Using ROLLBACK

The ROLLBACK statements end the present transaction and undo any change made during the transaction. The Rolling back is helpful for two reasons. Firstly, if you made a mistake such as deleting the wrong row from the table, rollbacks restore the original data. Secondly, if you start the transaction that you cannot finish as an exception is raised or the SQL statement fails the rollback return to the starting point to take the correct action and possibly try again.

Consider the illustration shown below, in which you insert information about an employee into 3 various database tables. All 3 tables have a column that hold employee numbers and is constrained by an exclusive index. If an INSERT statement tries to store a duplicate employee number, the predefined exception DUP_VAL_ON_ INDEX is raised. In that situation, you want to undo all the changes, so you issue a rollback in an exception handler.

DECLARE

emp_id INTEGER;

...

BEGIN

SELECT empno, ... INTO emp_id, ... FROM new_emp WHERE ...

...

INSERT INTO emp VALUES (emp_id, ...);

INSERT INTO tax VALUES (emp_id, ...);

INSERT INTO pay VALUES (emp_id, ...);

...

EXCEPTION

WHEN DUP_VAL_ON_INDEX THEN

ROLLBACK;

...

END;