More Oracle 9i Database Objects

This week we will be learing about Sequences, Synonyms, Indexes, Views, Data Dictionary View,
Controlling Database Objects, GRANT, REVOKE Roles and Privileges.

COMMIT, ROLLBACK and SAVEPOINT

SQL*PLUS has a facility to automatically commit all the work, without explicitly issuing the commit command i.e. SET AUTOCOMMIT ON -- Enables autocommit feature.

SET AUTOCOMMIT OFF -- Is the default and disables the automatic committing

Note: Certain actions automatically force a commit. DDL Commands CREATE, DROP, ALTER and DCL commands EXIT, GRANT, REVOKE, CONNECT, DISCONNECT, AUDIT and NOAUDIT are example for this. (i.e. the work done by these commands can not be taken back by ROLLBACK command).

SAVEPOINT: This statement is used to mark a point in the transaction to which you can later ROLLBACK. (i.e. Undo the changes made to the database till that point)

Following screen shots explain the usage of SAVEPOINT and ROLLBACK commands

RENAME COMMAND: This Command is used to change the name of a table.

SQL > RENAME STUD TO STUDENT;

LOCK TABLE : The LOCK TABLE command is used to prevent concurrent processes from changing a table or from using it.

The two Locking modes are
1)IN SHARE MODE: In which concurrent processes are allowed to perform only read-only operations.
2)IN EXCLUSIVE MODE:Prevents concurrent processes from performing any operation the table.

Only the Owner of the table, DBA, or a user having ALTER, DELETE, INSERT, SELECT, UPDATE can lock a table. A table can have multiple SHARE LOCKS but only one EXCLUSIVE LOCK

Example: For obtaining EXCLUSIVE LOCK we write.

SQL> LOCK TABLE STUD in EXCLUSIVE MODE;

SQL> LOCK TABLE STUD in EXCLUSIVE MODE NOWAIT;

If we try to obtain a lock using the first statement then Oracle waits if the table is not available for locking. In second statement it returns immediately.
SQL> LOCK TABLE STUD in SHARE MODE; --- to obtain shared mode lock

MORE DATABASE OBJECTS: A schema is a collection of components and database objects under the control of a given database user. Database Objects in Oracle are SEQUENCE, SYNONYM, and INDEX etc.

SEQUENCE: A sequence is an object in oracle, which is used to generate a series of numbers. It can generate unique sequential numbers for using in a primary key column of a table.

Ads

CURRVAL and NEXTVAL are called pseudo columns. CURRVAL returns the current value of a sequence. NEXTVAL increments the sequence and then returns the next value. A sequence can be modified using ALTER statement.

SQL> ALTER SEQUENCE SS

MAXVALUE 99999;

SQL> DROP SEQUENCE SS;