DBMS Questions and Answers

1. What is database?                                                                                          
 A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose.

2. What is DBMS?
It is a collection of programs that enables user to create and maintain a database. In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications.

3. What is a Database system?
The database and DBMS software together is called as Database system.

4. Advantages of DBMS?
Ø Redundancy is controlled.
Ø Unauthorised access is restricted.
Ø Providing multiple user interfaces.
Ø Enforcing integrity constraints.
Ø Providing backup and recovery.

5. Disadvantage in File Processing System?
Ø Data redundancy & inconsistency.
Ø Difficult in accessing data.
Ø Data isolation.
Ø Data integrity.
Ø Concurrent access is not possible.
Ø Security Problems.

6. Describe the three levels of data abstraction?
The are three levels of abstraction:
Ø Physical level: The lowest level of abstraction describes how data are stored.
Ø Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data.
Ø View level: The highest level of abstraction describes only part of entire database
7. Define the "integrity rules"
There are two Integrity rules.
Ø Entity Integrity: States that “Primary key cannot have NULL value”
Ø Referential Integrity: States that “Foreign Key can be either a NULL value or should be Primary Key value of other relation.

8. What is extension and intension?
Extension - It is the number of tuples present in a table at any instance. This is time dependent.
Intension - It is a constant value that gives the name, structure of table and the constraints laid on it.

9. What is System R? What are its two major subsystems?
System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.
Its two subsystems are
Ø Research Storage
Ø System Relational Data System.

10. How is the data structure of System R different from the relational structure?
Unlike Relational systems in System R
Ø Domains are not supported
Ø Enforcement of candidate key uniqueness is optional
Ø Enforcement of entity integrity is optional
Ø Referential integrity is not enforced

11. What is Data Independence?
Data independence means that “the application is independent of the storage structure and access strategy of data”. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.
Two types of Data Independence:
Ø Physical Data Independence: Modification in physical level should not affect the logical level.
Ø Logical Data Independence: Modification in logical level should affect the view level.
NOTE: Logical Data Independence is more difficult to achieve

12. What is a view? How it is related to data independence?
A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary.
Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence.

13. What is Data Model?
A collection of conceptual tools for describing data, data relationships data semantics and constraints.

14. What is E-R model?
This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set of attributes.

15. What is Object Oriented model?
This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes.

16. What is an Entity?
It is a 'thing' in the real world with an independent existence.

17. What is an Entity type?
It is a collection (set) of entities that have same attributes.
18. What is an Entity set?
It is a collection of all entities of particular entity type in the database.

19. What is an Extension of entity type?
The collections of entities of a particular entity type are grouped together into an entity set.

20. What is Weak Entity set?
An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.

21. What is an attribute?
It is a particular property, which describes the entity.

22. What is a Relation Schema and a Relation?
A relation Schema denoted by R(A1, A2, …, An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of n-values t=(v1,v2, ..., vn).

23. What is degree of a Relation?
It is the number of attribute of its relation schema.

24. What is Relationship?
It is an association among two or more entities.

25. What is Relationship set?
The collection (or set) of similar relationships.

26. What is Relationship type?
Relationship type defines a set of associations or a relationship set among a given set of entity types.

27. What is degree of Relationship type?
It is the number of entity type participating.

25. What is DDL (Data Definition Language)?
A data base schema is specifies by a set of definitions expressed by a special language called DDL.

26. What is VDL (View Definition Language)?
It specifies user views and their mappings to the conceptual schema

27. What is SDL (Storage Definition Language)?
This language is to specify the internal schema. This language may specify the mapping between two schemas.

28. What is Data Storage - Definition Language?
The storage structures and access methods used by database system are specified by a set of definition in a special type of DDL called data storage-definition language.

29. What is DML (Data Manipulation Language)?
This language that enable user to access or manipulate data as organised by appropriate data model.
Ø Procedural DML or Low level: DML requires a user to specify what data are needed and how to get those data.
Ø Non-Procedural DML or High level: DML requires a user to specify what data are needed without specifying how to get those data.

31. What is DML Compiler?
It translates DML statements in a query language into low-level instruction that the query evaluation engine can understand.

32. What is Query evaluation engine?
It executes low-level instruction generated by compiler.

33. What is DDL Interpreter?
It interprets DDL statements and record them in tables containing metadata.

34. What is Record-at-a-time?
The Low level or Procedural DML can specify and retrieve each record from a set of records. This retrieve of a record is said to be Record-at-a-time.

35. What is Set-at-a-time or Set-oriented?
The High level or Non-procedural DML can specify and retrieve many records in a single DML statement. This retrieve of a record is said to be Set-at-a-time or Set-oriented.

36. What is Relational Algebra?
It is procedural query language. It consists of a set of operations that take one or two relations as input and produce a new relation.

37. What is Relational Calculus?
It is an applied predicate calculus specifically tailored for relational databases proposed by E.F. Codd. E.g. of languages based on it are DSL ALPHA, QUEL.

38. How does Tuple-oriented relational calculus differ from domain-oriented relational calculus
The tuple-oriented calculus uses a tuple variables i.e., variable whose only permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables that range over the underlying domains instead of over relation. E.g. ILL, DEDUCE.

39. What is normalization?
It is a process of analysing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties
Ø Minimizing redundancy
Ø Minimizing insertion, deletion and update anomalies.

40. What is Functional Dependency?
A Functional dependency is denoted by X Y between two sets of attributes X and Y that are subsets of R specifies a constraint on the possible tuple that can form a relation state r of R. The constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This means the value of X component of a tuple uniquely determines the value of component Y.

41. When is a functional dependency F said to be minimal?
Ø Every dependency in F has a single attribute for its right hand side.
Ø We cannot replace any dependency X A in F with a dependency Y A where Y is a proper subset of X and still have a set of dependency that is equivalent to F.
Ø We cannot remove any dependency from F and still have set of dependency that is equivalent to F.

42. What is Multivalued dependency?
Multivalued dependency denoted by X Y specified on relation schema R, where X and Y are both subsets of R, specifies the following constraint on any relation r of R: if two tuples t1 and t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r with the following properties
Ø t3[x] = t4[X] = t1[X] = t2[X]
Ø t3[Y] = t1[Y] and t4[Y] = t2[Y]
Ø t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]

43. What is Lossless join property?
It guarantees that the spurious tuple generation does not occur with respect to relation schemas after decomposition.

44. What is 1 NF (Normal Form)?
The domain of attribute must include only atomic (simple, indivisible) values.

45. What is Fully Functional dependency?
It is based on concept of full functional dependency. A functional dependency X Y is full functional dependency if removal of any attribute A from X means that the dependency does not hold any more.

46. What is 2NF?
A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully functionally dependent on primary key.

47. What is 3NF?
A relation schema R is in 3NF if it is in 2NF and for every FD X A either of the following is true
Ø X is a Super-key of R.
Ø A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively dependent on primary key.

48. What is BCNF (Boyce-Codd Normal Form)?
A relation schema R is in BCNF if it is in 3NF and satisfies an additional constraint that for every FD X A, X must be a candidate key.

49. What is 4NF?
A relation schema R is said to be in 4NF if for every Multivalued dependency X Y that holds over R, one of following is true
Ø X is subset or equal to (or) XY = R.
Ø X is a super key.

50. What is 5NF?
A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ..., Rn} that holds R, one the following is true
Ø Ri = R for some i.
Ø The join dependency is implied by the set of FD, over R in which the left side is key of R.

51. What is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and dependencies that should hold on the the constraint can be enforced by simply enforcing the domain constraint and key constraint on the relation.

52. What are partial, alternate,, artificial, compound and natural key?
Partial Key:
It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity. It is sometime called as Discriminator.
Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate Keys.
Artificial Key:
If no obvious key, either stand alone or compound is available, then the last resort is to simply create a key, by assigning a unique number to each record or occurrence. Then this is known as developing an artificial key.
Compound Key:
If no single data element uniquely identifies occurrences within a construct, then combining multiple elements to create a unique identifier for the construct is known as creating a compound key.
Natural Key:
When one of the data elements stored within a construct is utilized as the primary key, then it is called the natural key.

53. What is indexing and what are the different kinds of indexing?
Indexing is a technique for determining how quickly specific data can be found.
Types:
Ø Binary search style indexing
Ø B-Tree indexing
Ø Inverted list indexing
Ø Memory resident table
Ø Table indexing

54. What is system catalog or catalog relation? How is better known as?
A RDBMS maintains a description of all the data that it contains, information about every relation and index that it contains. This information is stored in a collection of relations maintained by the system called metadata. It is also called data dictionary.

55. What is meant by query optimization?
The phase that identifies an efficient execution plan for evaluating a query that has the least estimated cost is referred to as query optimization.

56. What is join dependency and inclusion dependency?
Join Dependency:
A Join dependency is generalization of Multivalued dependency.A JD {R1, R2, ..., Rn} is said to hold over a relation R if R1, R2, R3, ..., Rn is a lossless-join decomposition of R . There is no set of sound and complete inference rules for JD.
Inclusion Dependency:
An Inclusion Dependency is a statement of the form that some columns of a relation are contained in other columns. A foreign key constraint is an example of inclusion dependency.

57. What is durability in DBMS?
Once the DBMS informs the user that a transaction has successfully completed, its effects should persist even if the system crashes before all its changes are reflected on disk. This property is called durability.

58. What do you mean by atomicity and aggregation?
Atomicity:
Either all actions are carried out or none are. Users should not have to worry about the effect of incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.
Aggregation:
A concept which is used to model a relationship between a collection of entities and relationships. It is used when we need to express a relationship among relationships.

59. What is a Phantom Deadlock?
In distributed deadlock detection, the delay in propagating local information might cause the deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are called phantom deadlocks and they lead to unnecessary aborts.

60. What is a checkpoint and When does it occur?
A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the DBMS can reduce the amount of work to be done during restart in the event of subsequent crashes.

61. What are the different phases of transaction?
Different phases are
Ø Analysis phase
Ø Redo Phase
Ø Undo phase

62. What do you mean by flat file database?
It is a database in which there are no programs or user access languages. It has no cross-file capabilities but is user-friendly and provides user-interface management.

63. What is "transparent DBMS"?
It is one, which keeps its Physical Structure hidden from user.

64. Brief theory of Network, Hierarchical schemas and their properties
Network schema uses a graph data structure to organize records example for such a database management system is CTCG while a hierarchical schema uses a tree data structure example for such a system is IMS.

65. What is a query?
A query with respect to DBMS relates to user commands that are used to interact with a data base. The query language can be classified into data definition language and data manipulation language.

66. What do you mean by Correlated subquery?
Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent query. Depending on how the subquery is written, it can be executed once for the parent query or it can be executed once for each row returned by the parent query. If the subquery is executed for each row of the parent, this is called a correlated subquery.
A correlated subquery can be easily identified if it contains any references to the parent subquery columns in its WHERE clause. Columns from the subquery cannot be referenced anywhere else in the parent query. The following example demonstrates a non-correlated subquery.
E.g. Select * From CUST Where '10/03/1990' IN (Select ODATE From ORDER Where CUST.CNUM = ORDER.CNUM)

67. What are the primitive operations common to all record management systems?
Addition, deletion and modification. 87. How are exceptions handled in PL/SQL? Give some of the internal exceptions' name
PL/SQL exception handling is a mechanism for dealing with run-time errors encountered during procedure execution. Use of this mechanism enables execution to continue if the error is not severe enough to cause procedure termination.
The exception handler must be defined within a subprogram specification. Errors cause the program to raise an exception with a transfer of control to the exception-handler block. After the exception handler executes, control returns to the block in which the handler was defined. If there are no more executable statements in the block, control returns to the caller.
User-Defined Exceptions
PL/SQL enables the user to define exception handlers in the declarations area of subprogram specifications. User accomplishes this by naming an exception as in the following example:
ot_failure EXCEPTION;
In this case, the exception name is ot_failure. Code associated with this handler is written in the EXCEPTION specification area as follows:
EXCEPTION
when OT_FAILURE then
out_status_code := g_out_status_code;
out_msg := g_out_msg;
The following is an example of a subprogram exception:
EXCEPTION
when NO_DATA_FOUND then
g_out_status_code := 'FAIL';
RAISE ot_failure;
Within this exception is the RAISE statement that transfers control back to the ot_failure exception handler. This technique of raising the exception is used to invoke all user-defined exceptions.
System-Defined Exceptions
Exceptions internal to PL/SQL are raised automatically upon error. NO_DATA_FOUND is a system-defined exception. Table below gives a complete list of internal exceptions.

68. Name the buffer in which all the commands that are typed in are stored
‘Edit’ Buffer

69. What are the unary operations in Relational Algebra?
PROJECTION and SELECTION.

70. Are the resulting relations of PRODUCT and JOIN operation the same?
No.
PRODUCT: Concatenation of every row in one relation with every row in another.
JOIN: Concatenation of rows from one relation and related rows from another.

71. What is RDBMS KERNEL?
Two important pieces of RDBMS architecture are the kernel, which is the software, and the data dictionary, which consists of the system-level data structures used by the kernel to manage the database
You might think of an RDBMS as an operating system (or set of subsystems), designed specifically for controlling data access; its primary functions are storing, retrieving, and securing data. An RDBMS maintains its own list of authorized users and their associated privileges; manages memory caches and paging; controls locking for concurrent resource usage; dispatches and schedules user requests; and manages space usage within its table-space structures.

72. Name the sub-systems of a RDBMS
I/O, Security, Language Processing, Process Control, Storage Management, Logging and Recovery, Distribution Control, Transaction Control, Memory Management, Lock Management

73. Which part of the RDBMS takes care of the data dictionary? How
Data dictionary is a set of tables and database objects that is stored in a special area of the database and maintained exclusively by the kernel.

74. What is the job of the information stored in data-dictionary?
The information in the data dictionary validates the existence of the objects, provides access to them, and maps the actual physical storage location.

75. Not only RDBMS takes care of locating data it also
determines an optimal access path to store or retrieve the data

If X YZ then X Y.
Ø Union or Additive rule:
If {X Y, X Z} then X YZ.
Ø Pseudo Transitive rule :
If {X Y, WY Z} then WX Z.
Of these the first three are known as Amstrong Rules. They are sound because it is enough if a set of FDs satisfy these three. They are called complete because using these three rules we can generate the rest all inference rules.

76. How do you communicate with an RDBMS?
You communicate with an RDBMS using Structured Query Language (SQL)

77. Define SQL and state the differences between SQL and other conventional programming Languages
SQL is a nonprocedural language that is designed specifically for data access operations on normalized relational database structures. The primary difference between SQL and other conventional programming languages is that SQL statements specify what data operations should be performed rather than how to perform them.

78. Name the three major set of files on disk that compose a database in Oracle
There are three major sets of files on disk that compose a database. All the files are binary. These are
Ø Database files
Ø Control files
Ø Redo logs
The most important of these are the database files where the actual data resides. The control files and the redo logs support the functioning of the architecture itself.
All three sets of files must be present, open, and available to Oracle for any data on the database to be useable. Without these files, you cannot access the database, and the database administrator might have to recover some or all of the database using a backup, if there is one.

79. What is an Oracle Instance?
The Oracle system processes, also known as Oracle background processes, provide functions for the user processes—functions that would otherwise be done by the user processes themselves
Oracle database-wide system memory is known as the SGA, the system global area or shared global area. The data and control structures in the SGA are shareable, and all the Oracle background processes and user processes can use them.
The combination of the SGA and the Oracle background processes is known as an Oracle instance

80. What are the four Oracle system processes that must always be up and running for the database to be useable
The four Oracle system processes that must always be up and running for the database to be useable include DBWR (Database Writer), LGWR (Log Writer), SMON (System Monitor), and PMON (Process Monitor).

81. What are database files, control files and log files. How many of these files should a database have at least? Why?
Database Files
The database files hold the actual data and are typically the largest in size. Depending on their sizes, the tables (and other objects) for all the user accounts can go in one database file—but that's not an ideal situation because it does not make the database structure very flexible for controlling access to storage for different users, putting the database on different disk drives, or backing up and restoring just part of the database.
You must have at least one database file but usually, more than one files are used. In terms of accessing and using the data in the tables and other objects, the number (or location) of the files is immaterial.
The database files are fixed in size and never grow bigger than the size at which they were created
Control Files
The control files and redo logs support the rest of the architecture. Any database must have at least one control file, although you typically have more than one to guard against loss. The control file records the name of the database, the date and time it was created, the location of the database and redo logs, and the synchronization information to ensure that all three sets of files are always in step. Every time you add a new database or redo log file to the database, the information is recorded in the control files.
Redo Logs
Any database must have at least two redo logs. These are the journals for the database; the redo logs record all changes to the user objects or system objects. If any type of failure occurs, the changes recorded in the redo logs can be used to bring the database to a consistent state without losing any committed transactions. In the case of non-data loss failure, Oracle can apply the information in the redo logs automatically without intervention from the DBA.
The redo log files are fixed in size and never grow dynamically from the size at which they were created.

82. What is ROWID?
The ROWID is a unique database-wide physical address for every row on every table. Once assigned (when the row is first inserted into the database), it never changes until the row is deleted or the table is dropped.
The ROWID consists of the following three components, the combination of which uniquely identifies the physical storage location of the row.
Ø Oracle database file number, which contains the block with the rows
Ø Oracle block address, which contains the row
Ø The row within the block (because each block can hold many rows)
The ROWID is used internally in indexes as a quick means of retrieving rows with a particular key value. Application developers also use it in SQL statements as a quick way to access a row once they know the ROWID

PL/SQL internal exceptions.

83. What is Oracle Block? Can two Oracle Blocks have the same address?
Oracle "formats" the database files into a number of Oracle blocks when they are first created—making it easier for the RDBMS software to manage the files and easier to read data into the memory areas.
The block size should be a multiple of the operating system block size. Regardless of the block size, the entire block is not available for holding data; Oracle takes up some space to manage the contents of the block. This block header has a minimum size, but it can grow.
These Oracle blocks are the smallest unit of storage. Increasing the Oracle block size can improve performance, but it should be done only when the database is first created.
Each Oracle block is numbered sequentially for each database file starting at 1. Two blocks can have the same block address if they are in different database files.

84. What is database Trigger?
A database trigger is a PL/SQL block that can defined to automatically execute for insert, update, and delete statements against a table. The trigger can e defined to execute once for the entire statement or once for every row that is inserted, updated, or deleted. For any one table, there are twelve events for which you can define database triggers. A database trigger can call database procedures that are also written in PL/SQL.

85. Name two utilities that Oracle provides, which are use for backup and recovery.
Along with the RDBMS software, Oracle provides two utilities that you can use to back up and restore the database. These utilities are Export and Import.
The Export utility dumps the definitions and data for the specified part of the database to an operating system binary file. The Import utility reads the file produced by an export, recreates the definitions of objects, and inserts the data
If Export and Import are used as a means of backing up and recovering the database, all the changes made to the database cannot be recovered since the export was performed. The best you can do is recover the database to the time when the export was last performed.

86. What are stored-procedures? And what are the advantages of using them?.
Stored procedures are database objects that perform a user defined operation. A stored procedure can have a set of compound SQL statements. A stored procedure executes the SQL commands and returns the result to the client. Stored procedures are used to reduce network traffic. 101. What are cursors give different types of cursors.
PL/SQL uses cursors for all database information accesses statements. The language supports the use two types of cursors
Ø Implicit
Ø Explicit

88. Does PL/SQL support "overloading"? Explain
The concept of overloading in PL/SQL relates to the idea that you can define procedures and functions with the same name. PL/SQL does not look only at the referenced name, however, to resolve a procedure or function call. The count and data types of formal parameters are also considered.
PL/SQL also attempts to resolve any procedure or function calls in locally defined packages before looking at globally defined packages or internal functions. To further ensure calling the proper procedure, you can use the dot notation. Prefacing a procedure or function name with the package name fully qualifies any procedure or function reference.

89. Tables derived from the ERD
a) Are totally unnormalised
b) Are always in 1NF
c) Can be further denormalised
d) May have multi-valued attributes

(b) Are always in 1NF

91. A B C is a set of attributes. The functional dependency is as follows
AB -> B
AC -> C
C -> B
a) is in 1NF
b) is in 2NF
c) is in 3NF
d) is in BCNF

(a) is in 1NF since (AC)+ = { A, B, C} hence AC is the primary key. Since C B is a FD given, where neither C is a Key nor B is a prime attribute, this it is not in 3NF. Further B is not functionally dependent on key AC thus it is not in 2NF. Thus the given FDs is in 1NF.

92. In mapping of ERD to DFD
a) entities in ERD should correspond to an existing entity/store in DFD
b) entity in DFD is converted to attributes of an entity in ERD
c) relations in ERD has 1 to 1 correspondence to processes in DFD
d) relationships in ERD has 1 to 1 correspondence to flows in DFD

(a) entities in ERD should correspond to an existing entity/store in DFD

93. A dominant entity is the entity
a) on the N side in a 1 : N relationship
b) on the 1 side in a 1 : N relationship
c) on either side in a 1 : 1 relationship
d) nothing to do with 1 : 1 or 1 : N relationship

(b) on the 1 side in a 1 : N relationship

94. Select 'NORTH', CUSTOMER From CUST_DTLS Where REGION = 'N' Order By CUSTOMER Union Select 'EAST', CUSTOMER From CUST_DTLS Where REGION = 'E' Order By CUSTOMER
The above is
a) Not an error
b) Error - the string in single quotes 'NORTH' and 'SOUTH'
c) Error - the string should be in double quotes
d) Error - ORDER BY clause

(d) Error - the ORDER BY clause. Since ORDER BY clause cannot be used in UNIONS

95. What is Storage Manager?
It is a program module that provides the interface between the low-level data stored in database, application programs and queries submitted to the system.

96. What is Buffer Manager?
It is a program module, which is responsible for fetching data from disk storage into main memory and deciding what data to be cache in memory.

97. What is Transaction Manager?
It is a program module, which ensures that database, remains in a consistent state despite system failures and concurrent transaction execution proceeds without conflicting.

98. What is File Manager?
It is a program module, which manages the allocation of space on disk storage and data structure used to represent information stored on a disk.

99. What is Authorization and Integrity manager?
It is the program module, which tests for the satisfaction of integrity constraint and checks the authority of user to access data.

100. What are stand-alone procedures?
Procedures that are not part of a package are known as stand-alone because they independently defined. A good example of a stand-alone procedure is one written in a SQL*Forms application. These types of procedures are not available for reference from other Oracle tools. Another limitation of stand-alone procedures is that they are compiled at run time, which slows execution. 90. Spurious tuples may occur due to

i. Bad normalization
ii. Theta joins
iii. Updating tables from join
a) i & ii b) ii & iii
c) i & iii d) ii & iii

(a) i & iii because theta joins are joins made on keys that are not primary keys.

102. What is cold backup and hot backup (in case of Oracle)?
Ø Cold Backup:
It is copying the three sets of files (database files, redo logs, and control file) when the instance is shut down. This is a straight file copy, usually from the disk directly to tape. You must shut down the instance to guarantee a consistent copy.
If a cold backup is performed, the only option available in the event of data file loss is restoring all the files from the latest backup. All work performed on the database since the last backup is lost.
Ø Hot Backup:
Some sites (such as worldwide airline reservations systems) cannot shut down the database while making a backup copy of the files. The cold backup is not an available option.
So different means of backing up database must be used — the hot backup. Issue a SQL command to indicate to Oracle, on a tablespace-by-tablespace basis, that the files of the tablespace are to backed up. The users can continue to make full use of the files, including making changes to the data. Once the user has indicated that he/she wants to back up the tablespace files, he/she can use the operating system to copy those files to the desired backup destination.
The database must be running in ARCHIVELOG mode for the hot backup option.
If a data loss failure does occur, the lost database files can be restored using the hot backup and the online and offline redo logs created since the backup was done. The database is restored to the most consistent state without any loss of committed transactions.

103. What are Armstrong rules? How do we say that they are complete and/or sound The well-known inference rules for FDs
Ø Reflexive rule :
If Y is subset or equal to X then X Y.
Ø Augmentation rule:
If X Y then XZ YZ.
Ø Transitive rule:
If {X Y, Y Z} then X Z.
Ø Decomposition rule :

104. How can you find the minimal key of relational schema?
Minimal key is one which can identify each tuple of the given relation schema uniquely. For finding the minimal key it is required to find the closure that is the set of all attributes that are dependent on any given set of attributes under the given set of functional dependency.
Algo. I Determining X+, closure for X, given set of FDs F
1. Set X+ = X
2. Set Old X+ = X+
3. For each FD Y Z in F and if Y belongs to X+ then add Z to X+
4. Repeat steps 2 and 3 until Old X+ = X+

Algo.II Determining minimal K for relation schema R, given set of FDs F
1. Set K to R that is make K a set of all attributes in R
2. For each attribute A in K
a. Compute (K – A)+ with respect to F
b. If (K – A)+ = R then set K = (K – A)+

105. What do you understand by dependency preservation?
Given a relation R and a set of FDs F, dependency preservation states that the closure of the union of the projection of F on each decomposed relation Ri is equal to the closure of F. i.e.,
((PR1(F)) U … U (PRn(F)))+ = F+
if decomposition is not dependency preserving, then some dependency is lost in the decomposition.

106. What is meant by Proactive, Retroactive and Simultaneous Update.
Proactive Update:
The updates that are applied to database before it becomes effective in real world .
Retroactive Update:
The updates that are applied to database after it becomes effective in real world .
Simulatneous Update:
The updates that are applied to database at the same time when it becomes effective in real world .

107. What are the different types of JOIN operations?
Equi Join: This is the most common type of join which involves only equality comparisions. The disadvantage in this type of join is that there

PL/SQL internal exceptions.

Exception Name Oracle Error
CURSOR_ALREADY_OPEN ORA-06511
DUP_VAL_ON_INDEX ORA-00001
INVALID_CURSOR ORA-01001
INVALID_NUMBER ORA-01722
LOGIN_DENIED ORA-01017
NO_DATA_FOUND ORA-01403
NOT_LOGGED_ON ORA-01012
PROGRAM_ERROR ORA-06501
STORAGE_ERROR ORA-06500
TIMEOUT_ON_RESOURCE ORA-00051
TOO_MANY_ROWS ORA-01422
TRANSACTION_BACKED_OUT ORA-00061
VALUE_ERROR ORA-06502
ZERO_DIVIDE ORA-01476

In addition to this list of exceptions, there is a catch-all exception named OTHERS that traps all errors for which specific error handling has not been established.

KEYS:

1. SELECT AVG(SCOST) FROM SOFTWARE WHERE DEVIN = 'ORACLE';
2. SELECT PNAME,TRUNC(MONTHS_BETWEEN(SYSDATE,DOB)/12) "AGE", TRUNC(MONTHS_BETWEEN(SYSDATE,DOJ)/12) "EXPERIENCE" FROM PROGRAMMER;
3. SELECT PNAME FROM STUDIES WHERE COURSE = 'PGDCA';
4. SELECT MAX(SOLD) FROM SOFTWARE;
5. SELECT PNAME, DOB FROM PROGRAMMER WHERE DOB LIKE '%APR%';
6. SELECT MIN(CCOST) FROM STUDIES;
7. SELECT COUNT(*) FROM STUDIES WHERE COURSE = 'DCA';
8. SELECT SUM(SCOST*SOLD-DCOST) FROM SOFTWARE GROUP BY DEVIN HAVING DEVIN = 'C';
9. SELECT * FROM SOFTWARE WHERE PNAME = 'RAKESH';
10. SELECT * FROM STUDIES WHERE SPLACE = 'PENTAFOUR';
11. SELECT * FROM SOFTWARE WHERE SCOST*SOLD-DCOST > 5000;
12. SELECT CEIL(DCOST/SCOST) FROM SOFTWARE;
13. SELECT * FROM SOFTWARE WHERE SCOST*SOLD >= DCOST;
14. SELECT MAX(SCOST) FROM SOFTWARE GROUP BY DEVIN HAVING DEVIN = 'VB';
15. SELECT COUNT(*) FROM SOFTWARE WHERE DEVIN = 'ORACLE';
16. SELECT COUNT(*) FROM STUDIES WHERE SPLACE = 'PRAGATHI';
17. SELECT COUNT(*) FROM STUDIES WHERE CCOST BETWEEN 10000 AND 15000;
18. SELECT AVG(CCOST) FROM STUDIES;
19. SELECT * FROM PROGRAMMER WHERE PROF1 = 'C' OR PROF2 = 'C';
20. SELECT * FROM PROGRAMMER WHERE PROF1 IN ('C','PASCAL') OR PROF2 IN ('C','PASCAL');
21. SELECT * FROM PROGRAMMER WHERE PROF1 NOT IN ('C','C++') AND PROF2 NOT IN ('C','C++'); LEGEND :
PNAME – Programmer Name, SPLACE – Study Place, CCOST – Course Cost, DEVIN – Developed in, SCOST – Software Cost, DCOST – Development Cost, PROF1 – Proficiency 1
22. SELECT TRUNC(MAX(MONTHS_BETWEEN(SYSDATE,DOB)/12)) FROM PROGRAMMER WHERE SEX = 'M';
23. SELECT TRUNC(AVG(MONTHS_BETWEEN(SYSDATE,DOB)/12)) FROM PROGRAMMER WHERE SEX = 'F';
24. SELECT PNAME, TRUNC(MONTHS_BETWEEN(SYSDATE,DOJ)/12) FROM PROGRAMMER ORDER BY PNAME DESC;
25. SELECT PNAME FROM PROGRAMMER WHERE TO_CHAR(DOB,'MON') = TO_CHAR(SYSDATE,'MON');
26. SELECT COUNT(*) FROM PROGRAMMER WHERE SEX = 'F';
27. SELECT DISTINCT(PROF1) FROM PROGRAMMER WHERE SEX = 'M';
28. SELECT AVG(SAL) FROM PROGRAMMER;
29. SELECT COUNT(*) FROM PROGRAMMER WHERE SAL BETWEEN 5000 AND 7500;
30. SELECT * FROM PROGRAMMER WHERE PROF1 NOT IN ('C','C++','PASCAL') AND PROF2 NOT IN ('C','C++','PASCAL');
31. SELECT PNAME,TITLE,SCOST FROM SOFTWARE WHERE SCOST IN (SELECT MAX(SCOST) FROM SOFTWARE GROUP BY PNAME);
32.SELECT 'Mr.' || PNAME || ' - has ' || TRUNC(MONTHS_BETWEEN(SYSDATE,DOJ)/12) || ' years of experience' “Programmer” FROM PROGRAMMER WHERE SEX = 'M' UNION SELECT 'Ms.' || PNAME || ' - has ' || TRUNC (MONTHS_BETWEEN (SYSDATE,DOJ)/12) || ' years of experience' “Programmer” FROM PROGRAMMER WHERE SEX = 'F';

II . SCHEMA :
Table 1 : DEPT
DEPTNO (NOT NULL , NUMBER(2)), DNAME (VARCHAR2(14)),
LOC (VARCHAR2(13)
Table 2 : EMP
EMPNO (NOT NULL , NUMBER(4)), ENAME (VARCHAR2(10)),
JOB (VARCHAR2(9)), MGR (NUMBER(4)), HIREDATE (DATE),
SAL (NUMBER(7,2)), COMM (NUMBER(7,2)), DEPTNO (NUMBER(2))

MGR is the empno of the employee whom the employee reports to. DEPTNO is a foreign key.

QUERIES :

1. Find out the selling cost average for packages developed in Oracle.
2. Display the names, ages and experience of all programmers.
3. Display the names of those who have done the PGDCA course.
4. What is the highest number of copies sold by a package?
5. Display the names and date of birth of all programmers born in April.
6. Display the lowest course fee.
7. How many programmers have done the DCA course.
8. How much revenue has been earned through the sale of packages developed in C.
9. Display the details of software developed by Rakesh.
10. How many programmers studied at Pentafour.
11. Display the details of packages whose sales crossed the 5000 mark.
12. Find out the number of copies which should be sold in order to recover the development cost of each package.
13. Display the details of packages for which the development cost has been recovered.
14. What is the price of costliest software developed in VB?
15. How many packages were developed in Oracle ?
16. How many programmers studied at PRAGATHI?
17. How many programmers paid 10000 to 15000 for the course?
18. What is the average course fee?
19. Display the details of programmers knowing C.
20. How many programmers know either C or Pascal?
21. How many programmers don’t know C and C++?
22. How old is the oldest male programmer?
23. What is the average age of female programmers?
24. Calculate the experience in years for each programmer and display along with their names in descending order.

25. Who are the programmers who celebrate their birthdays during the current month?

26. How many female programmers are there?
27. What are the languages known by the male programmers?
28. What is the average salary?
29. How many people draw 5000 to 7500?
30. Display the details of those who don’t know C, C++ or Pascal.
31. Display the costliest package developed by each programmer.
32. Produce the following output for all the male programmers

SQL – QUERIES

I. SCHEMAS
Table 1 : STUDIES
PNAME (VARCHAR), SPLACE (VARCHAR), COURSE (VARCHAR), CCOST (NUMBER)

Table 2 : SOFTWARE
PNAME (VARCHAR), TITLE (VARCHAR), DEVIN (VARCHAR), SCOST (NUMBER), DCOST (NUMBER), SOLD (NUMBER)

Table 3 : PROGRAMMER
PNAME (VARCHAR), DOB (DATE), DOJ (DATE), SEX (CHAR), PROF1 (VARCHAR), PROF2 (VARCHAR), SAL (NUMBER)

FAQ In SQL

1. Which is the subset of SQL commands used to manipulate Oracle Database structures, including tables?
Data Definition Language (DDL)

2. What operator performs pattern matching?
LIKE operator

3. What operator tests column for the absence of data?
IS NULL operator

4. Which command executes the contents of a specified file?
START or @

5. What is the parameter substitution symbol used with INSERT INTO command?
&

6. Which command displays the SQL command in the SQL buffer, and then executes it?  Ans:RUN

7. What are the wildcards used for pattern matching?
_ for single character substitution and % for multi-character substitution

8. State true or false. EXISTS, SOME, ANY are operators in SQL.
True

9. State true or false. !=, <>, ^= all denote the same operation.
True

10. What are the privileges that can be granted on a table by a user to others?
Insert, update, delete, select, references, index, execute, alter, all

11. What command is used to get back the privileges offered by the GRANT command?
REVOKE

12. Which system tables contain information on privileges granted and privileges obtained?
USER_TAB_PRIVS_MADE, USER_TAB_PRIVS_RECD

13. Which system table contains information on constraints on all the tables created?
USER_CONSTRAINTS

14. TRUNCATE TABLE EMP;  DELETE FROM EMP;
Will the outputs of the above two commands differ?
Both will result in deleting all the rows in the table EMP.

15. What is the difference between TRUNCATE and DELETE commands?
TRUNCATE is a DDL command whereas DELETE is a DML command. Hence DELETE operation can be rolled back, but TRUNCATE operation cannot be rolled back. WHERE clause can be used with DELETE and not with TRUNCATE.

16. What command is used to create a table by copying the structure of another table?
Answer :
CREATE TABLE .. AS SELECT command
Explanation :
To copy only the structure, the WHERE clause of the SELECT command should contain a FALSE statement as in the following.
CREATE TABLE NEWTABLE AS SELECT * FROM EXISTINGTABLE WHERE 1=2;
If the WHERE condition is true, then all the rows or rows satisfying the condition will be copied to the new table.

17. What will be the output of the following query?
SELECT REPLACE(TRANSLATE(LTRIM(RTRIM('!! ATHEN !!','!'), '!'), 'AN', '**'),'*','TROUBLE') FROM DUAL;
TROUBLETHETROUBLE

18. What will be the output of the following query?
SELECT DECODE(TRANSLATE('A','1234567890','1111111111'), '1','YES', 'NO' );
Answer :
NO
Explanation :
The query checks whether a given string is a numerical digit.

19. What does the following query do?
SELECT SAL + NVL(COMM,0) FROM EMP;
This displays the total salary of all employees. The null values in the commission column will be replaced by 0 and added to salary.

20. Which date function is used to find the difference between two dates?
MONTHS_BETWEEN

21. Why does the following command give a compilation error?
DROP TABLE &TABLE_NAME;
Variable names should start with an alphabet. Here the table name starts with an '&' symbol.

22. What is the advantage of specifying WITH GRANT OPTION in the GRANT command?
The privilege receiver can further grant the privileges he/she has obtained from the owner to any other user.

23. What is the use of the DROP option in the ALTER TABLE command?
It is used to drop constraints specified on the table.

24. What is the value of ‘comm’ and ‘sal’ after executing the following query if the initial value of ‘sal’ is 10000?
UPDATE EMP SET SAL = SAL + 1000, COMM = SAL*0.1;
sal = 11000, comm = 1000

25. What is the use of DESC in SQL?
Answer :
DESC has two purposes. It is used to describe a schema as well as to retrieve rows from table in descending order.
Explanation :
The query SELECT * FROM EMP ORDER BY ENAME DESC will display the output sorted on ENAME in descending order.

26. What is the use of CASCADE CONSTRAINTS?
When this clause is used with the DROP command, a parent table can be dropped even when a child table exists.

27. Which function is used to find the largest integer less than or equal to a specific value?
FLOOR

28. What is the output of the following query?
SELECT TRUNC(1234.5678,-2) FROM DUAL;
1200

QUERIES

1. List all the employees who have at least one person reporting to them.
2. List the employee details if and only if more than 10 employees are present in department no 10.
3. List the name of the employees with their immediate higher authority.
4. List all the employees who do not manage any one.
5. List the employee details whose salary is greater than the lowest salary of an employee belonging to deptno 20.

Read more ...

Real time Queries for SQL

Create emp table in your Schema. Insert 10 values, Then try bellow queries.

How to retrive the second record in the table(sql)?

(select rownum, e.* from emp e WHERE ROWNUM < 3) MINUS (select rownum,
e.* from emp e WHERE ROWNUM < 2) ;

How to find out the records except first record in the table(sql)?

(select rownum, e.* from emp e) MINUS (select rownum,e.* from emp
e WHERE ROWNUM < 2) ;

or

select * from emp where rowid > (select min(rowid) from emp);

How to find out the records except last record in the table(sql)?

select * from EMP WHERE ROWNUM <> (select COUNT(*) from EMP);

How to find out the last record in the table(sql)?

select * from emp where rownum<(select COUNT(*) from EMP)
minus
select * from emp where rownum<(select COUNT(*)-1 from EMP);
(Performance wise its not consider above query)

or

select * from emp where rowid < (select max(rowid) from emp);


How to retrive last record in the given table?

select * from emp where rowid=(select max(rowid) from emp);

How to retrive first record in the given table?

select * from emp where rowid = (select min(rowid) from emp);

How to find out the randon records in table or work space?
select distinct decode(rownum, 1, empno,6,empno,7, empno,ename ) from
emp;

or

select * from (select rownum rn, e.* from emp e) where rn in (1, 5, 9);

Read more ...

Req: How to attach the empno number before predefined value in a particular column in employee table? Using Sequence .....

Create new table, or other wise copy existing table. Then Create Sequence. While Inserting data into database using bellow insert query. Every time empno before added " PREDIFINEDVALUE00 ".
ex: PREDIFINEDVALUE001
PREDIFINEDVALUE002
PREDIFINEDVALUE003
...
etc.

Queries:

CREATE TABLE EMP_ONE AS SELECT * FROM EMP;

CREATE SEQUENCE EMP_ONE_SEQ
MINVALUE 1
MAXVALUE 999999999999999999999999999
START WITH 1
INCREMENT BY 1
CACHE 20;

SELECT * FROM ALL_SEQUENCES WHERE SEQUENCE_NAME = 'EMP_ONE_SEQ';

SELECT EMP_ONE_SEQ.CURRVAL FROM dual;

INSERT INTO EMP_ONE SELECT 'PREDIFINEDVALUE00'|| EMP_ONE_SEQ.NEXTVAL, ENAME, JOB, MGR,
HIREDATE, SAL, COMM, DEPTNO FROM EMP;

Read more ...

Oracle/PLSQL: Synonyms

A synonym is an alternative name for objects such as tables, views,
sequences, stored procedures, and other database objects.


Creating or replacing a synonym


The syntax for creating a synonym is:


create [or replace]  [public]  synonym [schema .] synonym_name
for [schema .] object_name [@ dblink];


The or replace phrase allows you to recreate the synonym (if it already
exists) without having to issue a DROP synonym command.


The public phrase means that the synonym is a public synonym and is
accessible to all users. Remember though that the user must first have the
appropriate privileges to the object to use the synonym.


The schema phrase is the appropriate schema. If this phrase is omitted,
Oracle assumes that you are referring to your own schema.


The object_name phrase is the name of the object for which you are creating
the synonym. It can be one of the following:



table package

view materialized view

sequence java class schema
object

stored user-defined object
procedure

function synonym




For Example:


create public synonym suppliers
for app.suppliers;


This first example demonstrates how to create a synonym called suppliers.
Now, users of other schemas can reference the table called suppliers
without having to prefix the table name with the schema named app. For
example:


select * from suppliers;


If this synonym already existed and you wanted to redefine it, you could
always use the or replace phrase as follows:


create or replace public synonym suppliers
for app.suppliers;


Dropping a synonym


It is also possible to drop a synonym. The syntax for dropping a synonym
is:


drop [public] synonym [schema .] synonym_name [force];


The public phrase allows you to drop a public synonym. If you have
specified public, then you don't specify a schema.


The force phrase will force Oracle to drop the synonym even if it has
dependencies. It is probably not a good idea to use the force phrase as it
can cause invalidation of Oracle objects.


For Example:


drop public synonym suppliers;


This drop statement would drop the synonym called suppliers that we defined
earlier.

Read more ...

SQL: VIEWS

A view is, in essence, a virtual table. It does not physically exist.
Rather, it is created by a query joining one or more tables.


Creating a VIEW


The syntax for creating a VIEW is:


CREATE VIEW view_name AS
SELECT columns
FROM table
WHERE predicates;


For Example:


CREATE VIEW sup_orders AS
SELECT suppliers.supplier_id, orders.quantity, orders.price
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id
and suppliers.supplier_name = 'IBM';


This would create a virtual table based on the result set of the select
statement. You can now query the view as follows:


SELECT *
FROM sup_orders;


Updating a VIEW


You can update a VIEW without dropping it by using the following syntax:


CREATE OR REPLACE VIEW view_name AS
SELECT columns
FROM table
WHERE predicates;


For Example:


CREATE or REPLACE VIEW sup_orders AS
SELECT suppliers.supplier_id, orders.quantity, orders.price
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id
and suppliers.supplier_name = 'Microsoft';


Dropping a VIEW


The syntax for dropping a VIEW is:


DROP VIEW view_name;


For Example:


DROP VIEW sup_orders;


Frequently Asked Questions



Question: Can you update the data in a view?


Answer: A view is created by joining one or more tables. When you update
record(s) in a view, it updates the records in the underlying tables that
make up the view.


So, yes, you can update the data in a view providing you have the proper
privileges to the underlying tables.





Question: Does the view exist if the table is dropped from the database?


Answer: Yes, in Oracle, the view continues to exist even after one of the
tables (that the view is based on) is dropped from the database. However,
if you try to query the view after the table has been dropped, you will
receive a message indicating that the view has errors.


If you recreate the table (that you had dropped), the view will again be
fine.

Read more ...

SQL: Data Types

The following is a list of general SQL datatypes that may not be supported
by all relational databases.


|-----------+------------+--------------------------------------|
| Data Type | Syntax | Explanation (if applicable) |
|-----------+------------+--------------------------------------|
| integer | integer | |
|-----------+------------+--------------------------------------|
| smallint | smallint | |
|-----------+------------+--------------------------------------|
| numeric | numeric | Where p is a precision value; s is a |
| | (p,s) | scale value. For example, numeric |
| | | (6,2) is a number that has 4 digits |
| | | before the decimal and 2 digits after|
| | | the decimal. |
|-----------+------------+--------------------------------------|
| decimal | decimal | Where p is a precision value; s is a |
| | (p,s) | scale value. |
|-----------+------------+--------------------------------------|
| real | real | Single-precision floating point |
| | | number |
|-----------+------------+--------------------------------------|
| double | double | Double-precision floating point |
| precision | precision | number |
|-----------+------------+--------------------------------------|
| float | float(p) | Where p is a precision value. |
|-----------+------------+--------------------------------------|
| character | char(x) | Where x is the number of characters |
| | | to store. This data type is space |
| | | padded to fill the number of |
| | | characters specified. |
|-----------+------------+--------------------------------------|
| character | varchar2(x)| Where x is the number of characters |
| varying | | to store. This data type does NOT |
| | | space pad. |
|-----------+------------+--------------------------------------|
| bit | bit(x) | Where x is the number of bits to |
| | | store. |
|-----------+------------+--------------------------------------|
| bit | bit varying| Where x is the number of bits to |
| varying | (x) | store. The length can vary up to x. |
|-----------+------------+--------------------------------------|
| date | date | Stores year, month, and day values. |
|-----------+------------+--------------------------------------|
| time | time | Stores the hour, minute, and second |
| | | values. |
|-----------+------------+--------------------------------------|
| timestamp | timestamp | Stores year, month, day, hour, |
| | | minute, and second values. |
|-----------+------------+--------------------------------------|
| time with | time with | Exactly the same as time, but also |
| time zone | time zone | stores an offset from UTC of the time|
| | | specified. |
|-----------+------------+--------------------------------------|
| timestamp | timestamp | Exactly the same as timestamp, but |
| with time | with time | also stores an offset from UTC of the|
| zone | zone | time specified. |
|-----------+------------+--------------------------------------|
| year-month| | Contains a year value, a month value,|
| interval | | or both. |
|-----------+------------+--------------------------------------|
| day-time | | Contains a day value, an hour value, |
| interval | | a minute value, and/or a second |
| | | value. |
|-----------+------------+--------------------------------------|

Read more ...

SQL: COUNT Function

The COUNT function returns the number of rows in a query.


The syntax for the COUNT function is:


SELECT COUNT(expression)
FROM tables
WHERE predicates;


Note:


The COUNT function will only count those records in which the field in the
brackets is NOT NULL.


For example, if you have the following table called suppliers:


|-------------+----------------+------|
| Supplier_ID | Supplier_Name | State|
|-------------+----------------+------|
| 1 | IBM | CA |
|-------------+----------------+------|
| 2 | Microsoft | |
|-------------+----------------+------|
| 3 | NVIDIA | |
|-------------+----------------+------|





The result for this query will return 3.


Select COUNT(Supplier_ID) from suppliers;


While the result for the next query will only return 1, since there is only
one row in the suppliers table where the State field is NOT NULL.


Select COUNT(State) from suppliers;


Simple Example:


For example, you might wish to know how many employees have a salary that
is above $25,000 / year.


SELECT COUNT(*) as "Number of employees"
FROM employees
WHERE salary > 25000;


In this example, we've aliased the count(*) field as "Number of employees".
As a result, "Number of employees" will display as the field name when the
result set is returned.


Example using DISTINCT:


You can use the DISTINCT clause within the COUNT function.


For example, the SQL statement below returns the number of unique
departments where at least one employee makes over $25,000 / year.


SELECT COUNT(DISTINCT department) as "Unique departments"
FROM employees
WHERE salary > 25000;


Again, the count(DISTINCT department) field is aliased as "Unique
departments". This is the field name that will display in the result set.


Example using GROUP BY:


In some cases, you will be required to use a GROUP BY clause with the COUNT
function.


For example, you could use the COUNT function to return the name of the
department and the number of employees (in the associated department) that
make over $25,000 / year.


SELECT department, COUNT(*) as "Number of employees"
FROM employees
WHERE salary > 25000
GROUP BY department;


Because you have listed one column in your SELECT statement that is not
encapsulated in the COUNT function, you must use a GROUP BY clause. The
department field must, therefore, be listed in the GROUP BY section.


TIP: Performance Tuning


Since the COUNT function will return the same results regardless of what
NOT NULL field(s) you include as the COUNT function parameters (ie: within
the brackets), you can change the syntax of the COUNT function to COUNT(1)
to get better performance as the database engine will not have to fetch
back the data fields.


For example, based on the example above, the following syntax would result
in better performance:


SELECT department, COUNT(1) as "Number of employees"
FROM employees
WHERE salary > 25000
GROUP BY department;


Now, the COUNT function does not need to retrieve all fields from the
employees table as it had to when you used the COUNT(*) syntax. It will
merely retrieve the numeric value of 1 for each record that meets your
criteria.





Practice Exercise #1:


Based on the employees table populated with the following data, count the
number of employees whose salary is over $55,000 per year.



CREATE
TABLE
employees

( employee_number number(10) not null,

employee_name varchar2(50) not null,

salary number(6),

CONSTRAINT
employees_pk
PRIMARY KEY
(employee_number)

);







INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1001, 'John Smith', 62000);


INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1002, 'Jane Anderson', 57500);


INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1003, 'Brad Everest', 71000);


INSERT INTO employees (employee_number, employee_name, salary)
VALUES (1004, 'Jack Horvath', 42000);


Solution:


Although inefficient in terms of performance, the following SQL statement
would return the number of employees whose salary is over $55,000 per year.


SELECT COUNT(*) as "Number of employees"
FROM employees
WHERE salary > 55000;


It would return the following result set:


|-----------------|
| Number of |
| employees |
|-----------------|
| 3 |
|-----------------|





A more efficient implementation of the same solution would be the following
SQL statement:


SELECT COUNT(1) as "Number of employees"
FROM employees
WHERE salary > 55000;


Now, the COUNT function does not need to retrieve all of the fields from
the table (ie: employee_number, employee_name, and salary), but rather
whenever the condition is met, it will retrieve the numeric value of 1.
Thus, increasing the performance of the SQL statement.





Practice Exercise #2:


Based on the suppliers table populated with the following data, count the
number of distinct cities in the suppliers table:



CREATE
TABLE
suppliers

( supplier_id number(10) not null,

supplier_name varchar2(50) not null,

city varchar2(50),

CONSTRAINT
suppliers_pk
PRIMARY KEY
(supplier_id)

);







INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5001, 'Microsoft', 'New York');


INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5002, 'IBM', 'Chicago');


INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5003, 'Red Hat', 'Detroit');


INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5004, 'NVIDIA', 'New York');


INSERT INTO suppliers (supplier_id, supplier_name, city)
VALUES (5005, 'NVIDIA', 'LA');


Solution:


The following SQL statement would return the number of distinct cities in
the suppliers table:


SELECT COUNT(DISTINCT city) as "Distinct Cities"
FROM suppliers;


It would return the following result set:


|-----------------|
| Distinct Cities |
|-----------------|
| 4 |
|-----------------|






Practice Exercise #3:


Based on the customers table populated with the following data, count the
number of distinct cities for each customer_name in the customers table:



CREATE
TABLE
customers

( customer_id number(10) not null,

customer_name varchar2(50) not null,

city varchar2(50),

CONSTRAINT
customers_pk
PRIMARY KEY
(customer_id)

);







INSERT INTO customers (customer_id, customer_name, city)
VALUES (7001, 'Microsoft', 'New York');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7002, 'IBM', 'Chicago');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7003, 'Red Hat', 'Detroit');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7004, 'Red Hat', 'New York');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7005, 'Red Hat', 'San Francisco');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7006, 'NVIDIA', 'New York');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7007, 'NVIDIA', 'LA');


INSERT INTO customers (customer_id, customer_name, city)
VALUES (7008, 'NVIDIA', 'LA');


Solution:


The following SQL statement would return the number of distinct cities for
each customer_name in the customers table:


SELECT customer_name, COUNT(DISTINCT city) as "Distinct Cities"
FROM customers
GROUP BY customer_name;


It would return the following result set:


|-------------------+------------|
| CUSTOMER_NAME | Distinct |
| | Cities |
|-------------------+------------|
| IBM | 1 |
|-------------------+------------|
| Microsoft | 1 |
|-------------------+------------|
| NVIDIA | 2 |
|-------------------+------------|
| Red Hat | 3 |
|-------------------+------------|

Read more ...

ROWID and ROWNUM in SQL

ROWNUM and ROWID are both referred to as pseudo-columns. That is, they are
not "real" columns that will show up when you DESC a table. They don't
actually exist anywhere in the database. But they're available for you to
use.

In fact, ROWNUM only exists for a row once it is retrieved from a query. It
represents the sequential order in which Oracle has retrieved the row.
Therefore it will always exist, be at least 1, and be unique (among the
rows returned by the query). Obviously it will change from query-to-query.
Let's look at a quick example:

scott@Robert> SELECT ROWNUM, ENAME, SAL
2 FROM EMP;

ROWNUM ENAME SAL
---------- ---------- ----------
1 SMITH 800
2 ALLEN 1600
3 WARD 1250
4 JONES 2975
5 MARTIN 1250
6 BLAKE 2850
7 CLARK 2450
8 SCOTT 3000
9 VOLLMAN 5000
10 TURNER 1500
11 ADAMS 1100
12 JAMES 950
13 FORD 3000
14 MILLER 1300


Ok so let's say we want the 5 highest paid employees. Should be easy:

scott@Robert> SELECT ROWNUM, ENAME, SAL
2 FROM EMP
3 WHERE ROWNUM < 6
4 ORDER BY SAL DESC;

ROWNUM ENAME SAL
---------- ---------- ----------
4 JONES 2975
2 ALLEN 1600
3 WARD 1250
5 MARTIN 1250
1 SMITH 800


Whoops! Turns out ROWNUM is assigned before results are ordered, not after.
Knowing that, we can write it like this:

scott@Robert> SELECT ENAME, SAL
2 FROM (SELECT ENAME, SAL FROM EMP ORDER BY SAL DESC) E
3 WHERE ROWNUM < 6;

ENAME SAL
---------- ----------
VOLLMAN 5000
SCOTT 3000
FORD 3000
JONES 2975
BLAKE 2850


What about ROWID? ROWID actually represents the physical location of the
record/row in the database. That being the case, it is (according to Oracle
documentation) the fastest way to retrieve a particular row. Faster than an
index, even.

Can you use ROWID to differentiate between duplicate rows?
Yes, you can. Since it actually represents the physical location of a row,
no two rows within the same table will have the same ROWID. Notice the
caveat I added: within the same table. If you're using clustering, two
records from different tables could theoretically share the same ROWID.

Do ROWIDs change?
Yes, especially with index organized or partitioned tables. Because ROWIDs
represent the physical location of a record/row, the ROWID will change
every time the record is physically moved.

Can you use ROWID as a primary key?
No, that's not advisable. While the ROWID will be unique, you would ideally
want to use a primary key that doesn't change.

How do you use ROWID to figure out what was the last record that was
processed?
Using DBMS_SQL.LAST_ROW_ID to get the ROWID of the last row processed.

You'll see ROWNUM and ROWID pop up occasionally within solutions to
problems on AskTom and various Discussion Forums, so I recommend adding it
to your own toolbelt as well.

Scenario 1:

You want to know why this is not working:
delete employee where rownum not in (select min(rownum) from employee group
by name,eno)

Ans: 1. Rowid is totally different from the rownum that you used in your
query.

2. Rowid is Unique for a record in the table (In the schema
itself).While you can't believe rownum to work like rowid(It may vary from
quey to query for the same table).

3. That's why this works fine:

SQL> Delete from employee where rowid not in (select min(rowid) from
employee group by name,eno);

If you are still confuse rownum and rowid click bellow link:

http://www.oracle.com/technetwork/issue-archive/2006/06-sep/o56asktom-086197.html

Read more ...

Oracle/PLSQL: Sequences (Autonumber)

In Oracle, you can create an autonumber field by using sequences. A
sequence is an object in Oracle that is used to generate a number sequence.
This can be useful when you need to create a unique number to act as a
primary key.


The syntax for a sequence is:


CREATE SEQUENCE sequence_name
    MINVALUE value
    MAXVALUE value
    START WITH value
    INCREMENT BY value
    CACHE value;


For example:


CREATE SEQUENCE supplier_seq
    MINVALUE 1
    MAXVALUE 999999999999999999999999999
    START WITH 1
    INCREMENT BY 1
    CACHE 20;


This would create a sequence object called supplier_seq. The first sequence
number that it would use is 1 and each subsequent number would increment by
1 (ie: 2,3,4,...}. It will cache up to 20 values for performance.


If you omit the MAXVALUE option, your sequence will automatically default
to:


MAXVALUE 999999999999999999999999999


So you can simplify your CREATE SEQUENCE command as follows:


CREATE SEQUENCE supplier_seq
    MINVALUE 1
    START WITH 1
    INCREMENT BY 1
    CACHE 20;


Now that you've created a sequence object to simulate an autonumber field,
we'll cover how to retrieve a value from this sequence object. To retrieve
the next value in the sequence order, you need to use nextval.


For example:


supplier_seq.nextval


This would retrieve the next value from supplier_seq. The nextval statement
needs to be used in an SQL statement. For example:


INSERT INTO suppliers
(supplier_id, supplier_name)
VALUES
(supplier_seq.nextval, 'Kraft Foods');


This insert statement would insert a new record into the suppliers table.
The supplier_id field would be assigned the next number from the
supplier_seq sequence. The supplier_name field would be set to Kraft Foods.


Frequently Asked Questions in Sequences



One common question about sequences is:


Question: While creating a sequence, what does cache and nocache options
mean? For example, you could create a sequence with a cache of 20 as
follows:


CREATE SEQUENCE supplier_seq
    MINVALUE 1
    START WITH 1
    INCREMENT BY 1
    CACHE 20;





Or you could create the same sequence with the nocache option:


CREATE SEQUENCE supplier_seq
    MINVALUE 1
    START WITH 1
    INCREMENT BY 1
    NOCACHE;





Answer: With respect to a sequence, the cache option specifies how many
sequence values will be stored in memory for faster access.


The downside of creating a sequence with a cache is that if a system
failure occurs, all cached sequence values that have not be used, will be
"lost". This results in a "gap" in the assigned sequence values. When the
system comes back up, Oracle will cache new numbers from where it left off
in the sequence, ignoring the so called "lost" sequence values.


Note: To recover the lost sequence values, you can always execute an
ALTER SEQUENCE command to reset the counter to the correct value.


Nocache means that none of the sequence values are stored in memory. This
option may sacrifice some performance, however, you should not encounter a
gap in the assigned sequence values.



Question: How do we set the LASTVALUE value in an Oracle Sequence?


Answer: You can change the LASTVALUE for an Oracle sequence, by executing
an ALTER SEQUENCE command.


For example, if the last value used by the Oracle sequence was 100 and you
would like to reset the sequence to serve 225 as the next value. You would
execute the following commands.


alter sequence seq_name
increment by 124;


select seq_name.nextval from dual;


alter sequence seq_name
increment by 1;


Now, the next value to be served by the sequence will be 225.

Read more ...

Enum: JPA and Enums via @Enumerated

JPA and Enums via @Enumerated

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It can sometimes be desirable to have a Java enum type to represent a particular column in a database. JPA supports converting database data to and from Java enum types via the @javax.persistence.Enumerated annotation.

This example will show basic @Enumerated usage in a field of an @Entity as well as enums as the parameter of a Query. We'll also see that the actual database representation can be effectively String or int.

Enum

For our example we will leverage the familiar Movie entity and add a new field to represent the MPAA.org rating of the movie. This is defined via a simple enumthat requires no JPA specific annotations.

public enum Rating {
    UNRATED,
    G,
    PG,
    PG13,
    R,
    NC17
}

@Enumerated

In our Movie entity, we add a rating field of the enum type Rating and annotate it with @Enumerated(EnumType.STRING) to declare that its value should be converted from what is effectively a String in the database to the Rating type.

@Entity
public class Movie {

    @Id
    @GeneratedValue
    private int id;
    private String director;
    private String title;
    private int year;

    @Enumerated(EnumType.STRING)
    private Rating rating;

    public Movie() {
    }

    public Movie(String director, String title, int year, Rating rating) {
        this.director = director;
        this.title = title;
        this.year = year;
        this.rating = rating;
    }

    public String getDirector() {
        return director;
    }

    public void setDirector(String director) {
        this.director = director;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public Rating getRating() {
        return rating;
    }

    public void setRating(Rating rating) {
        this.rating = rating;
    }
}

The above is enough and we are effectively done. For the sake of completeness we'll show a sample Query

Enum in JPQL Query

Note the findByRating method which creates a Query with a rating named parameter. The key thing to notice is that the rating enum instance itself is passed into the query.setParameter method, not rating.name() or rating.ordinal().

Regardless if you use EnumType.STRING or EnumType.ORDINAL, you still always pass the enum itself in calls to query.setParameter.

@Stateful
public class Movies {

    @PersistenceContext(unitName = "movie-unit", type = PersistenceContextType.EXTENDED)
    private EntityManager entityManager;

    public void addMovie(Movie movie) {
        entityManager.persist(movie);
    }

    public void deleteMovie(Movie movie) {
        entityManager.remove(movie);
    }

    public List<Movie> findByRating(Rating rating) {
        final Query query = entityManager.createQuery("SELECT m FROM Movie as m WHERE m.rating = :rating");
        query.setParameter("rating", rating);
        return query.getResultList();
    }

    public List<Movie> getMovies() throws Exception {
        Query query = entityManager.createQuery("SELECT m from Movie as m");
        return query.getResultList();
    }

}

EnumType.STRING vs EnumType.ORDINAL

It is a matter of style how you would like your enum data represented in the database. Either name() or ordinal() are supported:

• @Enumerated(EnumType.STRING) Rating rating the value of rating.name() is written and read from the corresponding database column; e.g. G, PG,PG13
• @Enumerated(EnumType.ORDINAL) Rating rating the value of rating.ordinal() is written and read from the corresponding database column; e.g. 0,1, 2

The default is EnumType.ORDINAL

There are advantages and disadvantages to each.

Disadvantage of EnumType.ORDINAL

A disadvantage of EnumType.ORDINAL is the effect of time and the desire to keep enums in a logical order. With EnumType.ORDINAL any new enum elements must be added to the end of the list or you will accidentally change the meaning of all your records.

Let's use our Rating enum and see how it would have had to evolve over time to keep up with changes in the MPAA.org ratings system.

1980

public enum Rating {
    G,
    PG,
    R,
    UNRATED
}

1984 PG-13 is added

public enum Rating {
    G,
    PG,
    R,
    UNRATED,
    PG13
}

1990 NC-17 is added

public enum Rating {
    G,
    PG,
    R,
    UNRATED,
    PG13,
    NC17
}

If EnumType.STRING was used, then the enum could be reordered at anytime and would instead look as we have defined it originally with ratings starting at Gand increasing in severity to NC17 and eventually UNRATED. With EnumType.ORDINAL the logical ordering would not have withstood the test of time as new values were added.

If the order of the enum values is significant to your code, avoid EnumType.ORDINAL

Unit Testing the JPA @Enumerated

public class MoviesTest extends TestCase {

    public void test() throws Exception {

        final Properties p = new Properties();
        p.put("movieDatabase", "new://Resource?type=DataSource");
        p.put("movieDatabase.JdbcDriver", "org.hsqldb.jdbcDriver");
        p.put("movieDatabase.JdbcUrl", "jdbc:hsqldb:mem:moviedb");

        EJBContainer container = EJBContainer.createEJBContainer(p);
        final Context context = container.getContext();

        final Movies movies = (Movies) context.lookup("java:global/jpa-scratch/Movies");

        movies.addMovie(new Movie("James Frawley", "The Muppet Movie", 1979, Rating.G));
        movies.addMovie(new Movie("Jim Henson", "The Great Muppet Caper", 1981, Rating.G));
        movies.addMovie(new Movie("Frank Oz", "The Muppets Take Manhattan", 1984, Rating.G));
        movies.addMovie(new Movie("James Bobin", "The Muppets", 2011, Rating.PG));

        assertEquals("List.size()", 4, movies.getMovies().size());

        assertEquals("List.size()", 3, movies.findByRating(Rating.G).size());

        assertEquals("List.size()", 1, movies.findByRating(Rating.PG).size());

        assertEquals("List.size()", 0, movies.findByRating(Rating.R).size());

        container.close();
    }
}

Running

To run the example via maven:

cd jpa-enumerated
mvn clean install

Which will generate output similar to the following:

-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running org.superbiz.jpa.enums.MoviesTest
Apache OpenEJB 4.0.0-beta-2    build: 20120115-08:26
http://tomee.apache.org/
INFO - openejb.home = /Users/dblevins/openejb/examples/jpa-enumerated
INFO - openejb.base = /Users/dblevins/openejb/examples/jpa-enumerated
INFO - Using 'javax.ejb.embeddable.EJBContainer=true'
INFO - Configuring Service(id=Default Security Service, type=SecurityService, provider-id=Default Security Service)
INFO - Configuring Service(id=Default Transaction Manager, type=TransactionManager, provider-id=Default Transaction Manager)
INFO - Configuring Service(id=movieDatabase, type=Resource, provider-id=Default JDBC Database)
INFO - Found EjbModule in classpath: /Users/dblevins/openejb/examples/jpa-enumerated/target/classes
INFO - Beginning load: /Users/dblevins/openejb/examples/jpa-enumerated/target/classes
INFO - Configuring enterprise application: /Users/dblevins/openejb/examples/jpa-enumerated
INFO - Configuring Service(id=Default Stateful Container, type=Container, provider-id=Default Stateful Container)
INFO - Auto-creating a container for bean Movies: Container(type=STATEFUL, id=Default Stateful Container)
INFO - Configuring Service(id=Default Managed Container, type=Container, provider-id=Default Managed Container)
INFO - Auto-creating a container for bean org.superbiz.jpa.enums.MoviesTest: Container(type=MANAGED, id=Default Managed Container)
INFO - Configuring PersistenceUnit(name=movie-unit)
INFO - Auto-creating a Resource with id 'movieDatabaseNonJta' of type 'DataSource for 'movie-unit'.
INFO - Configuring Service(id=movieDatabaseNonJta, type=Resource, provider-id=movieDatabase)
INFO - Adjusting PersistenceUnit movie-unit  to Resource ID 'movieDatabaseNonJta' from 'movieDatabaseUnmanaged'
INFO - Enterprise application "/Users/dblevins/openejb/examples/jpa-enumerated" loaded.
INFO - Assembling app: /Users/dblevins/openejb/examples/jpa-enumerated
INFO - PersistenceUnit(name=movie-unit, provider=org.apache.openjpa.persistence.PersistenceProviderImpl) - provider time 406ms
INFO - Jndi(name="java:global/jpa-enumerated/Movies!org.superbiz.jpa.enums.Movies")
INFO - Jndi(name="java:global/jpa-enumerated/Movies")
INFO - Created Ejb(deployment-id=Movies, ejb-name=Movies, container=Default Stateful Container)
INFO - Started Ejb(deployment-id=Movies, ejb-name=Movies, container=Default Stateful Container)
INFO - Deployed Application(path=/Users/dblevins/openejb/examples/jpa-enumerated)
INFO - Undeploying app: /Users/dblevins/openejb/examples/jpa-enumerated
INFO - Closing DataSource: movieDatabase
INFO - Closing DataSource: movieDatabaseNonJta
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 2.831 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

Read more ...