SQL INTERVIEW QUESTIONS

1)      What are the components of Physical database structure of Oracle Database?

          ORACLE database is comprised of three types of files. One or more Data files, two are  more Redo Log files, and one or more Control files.

2)      What are the components of Logical database structure of ORACLE database?

           Table spaces and the Database's Schema Objects.

3)      What is a Table space?

          A database is divided into Logical Storage Unit called table spaces. A table space is used to     grouped related logical structures together.

4)      What is SYSTEM table space and when is it Created?

          Every ORACLE database contains a table space named SYSTEM, which is automatically created when the database is created. The SYSTEM table space always contains the data dictionary tables for the entire database.

5)      Explain the relationship among Database, Table space and Data file.

         Each databases logically divided into one or more table spaces one or more data files are    explicitly created for each table space.

6)      What is schema?

          A schema is collection of database objects of a User.

7)      What are Schema Objects?

          Schema objects are the logical structures that directly refer to the database's   data. Schema    objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.

8)      Can objects of the same Schema reside in different table space?

         Yes.

9)      Can a Table space hold objects from different Schemes?

          Yes.

10)   What is Table?

           A table is the basic unit of data storage in an ORACLE database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.

11)   What is a View?

          A view is a virtual table. Every view has a Query attached to it. (The Query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)

12)   Does View contain Data?

          Views do not contain or store data.

13)   Can a View based on another View?

          Yes.

14)   What are the advantages of Views?

           1. Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.

           2. Hide data complexity.

           3. Simplify commands for the user.

           4. Present the data in a different perspective from that of the base table.

           5. Store complex queries.

15)   What is a Sequence?

           A sequence generates a serial list of unique numbers for numerical columns of a      database's tables.

16)   What is a Synonym?

          A synonym is an alias for a table, view, sequence or program unit.

17)   What is the type of Synonyms?

          There are two types of Synonyms Private and Public.

18)   What is a Private Synonyms?

          A Private Synonyms can be accessed only by the owner.

19)   What is a Public Synonym?

          Public synonyms can be accessed by any user on the database.

20)   What are synonyms used for?

·        Synonyms are used to: Mask the real name and owner of an object.

·        Provide public access to an object.

·        Provide location transparency for tables, views or program units of a remote database.

·        Simplify the SQL statements for database users.

21)   What is an Index?

          An Index is an optional structure associated with a table to have direct access to rows,   which can be created to increase the performance of data retrieval.  Index can be created on one or more columns of a table.

22)   How is Indexes Update?

           Indexes are automatically maintained and used by ORACLE. Changes to table data   are       automatically incorporated into all relevant indexes.

23)   What are Clusters?

           Clusters are groups of one or more tables physically stores together to share common columns and are often used together.

24)   What is cluster Key?

          The related column of the tables in a cluster is called the Cluster Key.

25)   What is Index Cluster?

          A Cluster with an index on the Cluster Key.

26)   What is Hash Cluster?

            A row is stored in a hash cluster based on the result of applying a hash function to the row’s cluster key value. All rows with the same hash key value are stores together on disk.

27)   When can Hash Cluster used?

           Hash clusters are better choice when a table is often queried with equality queries.  For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.

28)   What is Database Link?

          A database link is a named object that describes a "path" from one database to another.

29)   What are the types of Database Links?

          Private Database Link, Public Database Link & Network Database Link.

30)   What is Private Database Link?

           Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner's views or procedures.

31)   What is Public Database Link?

           Public database link is created for the special user group PUBLIC. A public database  link  can  be  used  when  any  user  in  the associated database specifies a global object name in a SQL  statement or object definition.

32)   What is Network Database link?

           Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.

33)   What is Data Block?

           ORACLE database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.

34)   How to define Data Block size?

           A data block size is specified for each ORACLE database when the database is created.  A database users and allocated free database space in ORACLE data blocks.  Block size is specified in INIT.ORA file and can’t be changed latter.

35)   What is Row Chaining?

           In Circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment.

36)   What is an Extent?

           An Extent is a specific number of contiguous data blocks, obtained in a single allocation, and used to store a specific type of information.

37)   What is a Segment?

           A segment is a set of extents allocated for a certain logical structure.

38)   What are the different types of Segments?

           Data Segment, Index Segment, Rollback Segment and Temporary Segment.

39)   What is a Data Segment?

           Each Non-clustered table has a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster's data segment.

40)   What is an Index Segment?

          Each Index has an Index segment that stores all of its data.

41)   What is Rollback Segment?

          A  Database contains one or more Rollback Segments to temporarily store "undo" information.

42)   What are the uses of Rollback Segment?

           Rollback Segments are used: To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.

43)   What is a Temporary Segment?

           Temporary segments are created by ORACLE when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.

44)   What is a Data File?

           Every ORACLE database has one or more physical data files. A database's data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.

45)   What are the Characteristics of Data Files?

·        A data file can be associated with only one database.

·        Once created a data file can't change size.

·        One or more data files form a logical unit of database storage called a table space.

46)   What is a Redo Log?

          The set of Redo Log files for a database is collectively known as the database's redo log.

47)   What is the function of Redo Log?

          The Primary function of the redo log is to record all changes made to data.

48)   What is the use of Redo Log Information?

           The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files.

49)   What does a Control file Contain?

           A Control file records the physical structure of the database. It contains the following information.

           Database Name

1.      Names and locations of a database's files and redo log files.

2.      Time stamp of database creation.

50)   What is the use of Control File?

           When an instance of an ORACLE database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.

51)   What is a Data Dictionary?

                      i. The data dictionary of an ORACLE database is a set of tables and views that are used as a read-only reference about the database.

                    ii. It stores information about both the logical and physical structure of the database, the valid users of an ORACLE database, integrity constraints defined for tables in the database and space allocated for a schema object and how much of it is being used.

52)   What is an Integrity Constrains?

           An integrity constraint is a declarative way to define a business rule for a column of a table.

53)   Can an Integrity Constraint be enforced on a table if some existing table data does  not satisfy the constraint?

           No.

54)   Describe the different type of Integrity Constraints supported by ORACLE?

·        NOT NULL Constraint - Disallows Nulls in a table's column.

·        UNIQUE Constraint - Disallows duplicate values in a column or set of columns.

·        PRIMARY KEY Constraint - Disallows duplicate values and Nulls in a column or set of columns.

·        FOREIGN KEY Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or PRIMARY KEY.

·        CHECK Constraint - Disallows values that do not satisfy the logical expression of the constraint.

 55)   What is difference between UNIQUE constraint and PRIMARY KEY constraint?

           A column defined as   UNIQUE can contain Nulls while a column defined as PRIMARY KEY can't contain Nulls.

56)   Describe Referential Integrity?

           A rule defined on a column (or set of columns) in one table that allows the insert or update of a  row  only  if the value for the column or set of columns  (the  dependent  value)  matches  a value in a column of a related table   (the  referenced  value).  It also specifies the type of data manipulation allowed on referenced data and the action to be performed on dependent data as a result of any action on referenced data.

57)   What are the Referential actions supported by FOREIGN KEY integrity constraint?

           UPDATE and DELETE Restrict - A referential integrity rule that disallows the update or deletion of referenced data.

           DELETE Cascade - When a referenced row is deleted all associated dependent rows are deleted.

58)   What is self-referential integrity constraint?

           If a foreign key reference a parent key of the same table is called self-referential integrity constraint.

59)   What are the Limitations of a CHECK Constraint?

           The  condition  must  be a Boolean expression evaluated using the values in the row being inserted or updated and can't contain subquery, sequence, the SYSDATE,UID,USER  or  USERENV  SQL functions, or the pseudo columns LEVEL or ROWNUM.

60)   What is the maximum number of CHECK constraints that can be defined on a column?

          No Limit.

61)   What constitute an ORACLE Instance?

           SGA and ORACLE background processes constitute an ORACLE instance. (Or) Combination of memory structure and background process.

62)   What is SGA?

           The System Global Area (SGA) is a shared memory region allocated by ORACLE that contains data and control information for one ORACLE instance.

63)   What are the components of SGA?

          Database buffers, Redo Log Buffer the Shared Pool and   Cursors.

64)   What is PGA?

           Program Global Area (PGA) is a memory buffer that contains data and control information for a server process.

              

65)   Define Transaction?

           A  Transaction is a logical unit of work that comprises one or more SQL statements executed by a single user.

66)   When does a Transaction end?

         When it is committed or Roll backed.

67)   What does COMMIT do?

           COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions transactions that start only after transaction is committed.

68)   What does ROLLBACK do?

         ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.

69)   What is SAVE POINT?

           For  long  transactions  that  contain  many  SQL  statements, intermediate markers  or  save points  can  be  declared  which  can  be used to divide a transaction  into  smaller  parts.  This allows the option of later rolling back all work performed from the current point in the transaction to a declared save point within the transaction.

70)   What is Read-Only Transaction?

           Read-Only transaction ensures that the results of each query executed in the transaction are consistent with respect to the same point in time.

71)   What is the function of Optimizer?

          The goal of the optimizer is to choose the most efficient way to execute a SQL statement.

72)   What is Execution Plan?

           The combination of the steps the optimizer chooses to execute a statement is called an execution plan.

73)   What are the different approaches used by Optimizer in choosing an execution plan?

          Rule-based and Cost-based.

74)   What are the factors that affect OPTIMIZER in choosing an Optimization approach?

           The   OPTIMIZER_MODE   initialization parameter Statistics in the Data Dictionary the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in the statement.

75)   What are the values that can be specified for OPTIMIZER MODE Parameter?

         COST and RULE.

76)   Will the Optimizer always use COST-based approach if OPTIMIZER_MODE is set to "Cost'?

           Presence of statistics in the data dictionary for at least one of the tables accessed by the SQL statements is necessary for the OPTIMIZER to use COST-based approach. Otherwise OPTIMIZER chooses RULE-based approach.

77)   What is the effect of setting the value of OPTIMIZER_MODE to ‘RULE’?

           This  value  causes the optimizer to choose the rule based approach for all SQL  statements  issued  to  the  instance  regardless  of  the presence of statistics.

78)   What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION Command?

          CHOOSE, ALL_ROWS, FIRST_ROWS and RULE.

79)   What is the effect of setting the value "CHOOSE" for OPTIMIZER_GOAL, parameter of the ALTER SESSION Command?

           The Optimizer chooses Cost based approach and optimizes with the goal of best throughput if statistics for at least one of the tables accessed by the SQL statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach.

80)   What is the effect of setting the value "ALL_ROWS” for OPTIMIZER_GOAL parameter of the ALTER SESSION command?

           This value causes the optimizer to the cost-based approach for all SQL statements in the session regardless of the presence of statistics and to optimize with a goal of best throughput.

81)   What   is   the effect of setting the value ‘FIRST_ROWS’ for OPTIMIZER_GOAL parameter of the ALTER SESSION command?

           This value causes the optimizer to use the cost-based approach for all SQL statements in the session regardless of the presence of statistics and to optimize with a goal of best response time.

82)   What is the effect of setting the 'RULE' for OPTIMIER_GOAL parameter of the ALTER SESSION Command?

           This value causes the optimizer to choose the rule-based approach for all SQL statements in a session regardless of the presence of statistics.

83)   What is RULE-based approach to optimization?

           Choosing an executing plan based on the access paths available and the ranks of   these access paths.

84)   What is COST-based approach to optimization?

           Considering  available  access  paths  and  determining  the most efficient execution  plan  based  on statistics in the data dictionary for the tables accessed by the statement and their associated  clusters and indexes.                              

85)   What are the different types of PL/SQL program units that can be defined and stored in ORACLE database?

          Procedures and Functions, Packages and Database Triggers.

86)   What is a Procedure?

           A Procedure consist of a set of SQL and PL/SQL statements that are grouped together  as a unit to solve a specific problem or perform a set of related tasks.

87)   What is difference between Procedures and Functions?

           A Function returns a value to the caller where as a Procedure does not.

88)   What is a Package?

           A Package is a collection of related procedures, functions, variables and other package constructs together as a unit in the database.

89)   What are the advantages of having a Package?

           Increased  functionality  (for  example, global  package  variables  can  be declared  and  used  by  any procedure in the package) and performance (for example  all  objects  of  the package are parsed compiled, and loaded into memory once)

90)   What is Database Trigger?

           A Database Trigger is procedure (set of SQL and PL/SQL statements) that is automatically executed as a result of an insert in, update to, or delete from a table.

91)   What are the uses of Database Trigger?

           Database triggers can be used to automatic data generation, audit data modifications, enforce complex Integrity constraints, and customize complex security authorizations.

92)   What are the differences between Database Trigger and Integrity constraints?

           A declarative integrity constraint is a statement about the database that is always true. A constraint applies to existing data in the table and any statement that manipulates the table.

           A  trigger  does  not  apply  to  data  loaded before the definition of the trigger,  therefore,  it does not guarantee all data in a table conforms to the rules established by an associated trigger.

           A trigger can be used to enforce transitional constraints where as a declarative integrity constraint cannot be used.

93)   What is a SQL * NET?

           SQL *NET is ORACLE’s mechanism for interfacing with the communication protocols used by the networks that facilitate distributed processing and distributed databases.  It is used in Clint-Server and Server-Server communications.