dbms

1. TRIGGER IN PL/SQL

2. TRIGGER
 A trigger is a stored PL/SQL program unit
that is automatically executed (fired) by
Oracle whenever a specified event occurs on
a table, view, schema, or database.

3. TRIGGERS ARE MAINLY USED
FOR:
 Enforcing business rules
 Auditing changes (history of
updates/deletes)
 Automatic actions (like logging, cascading
updates)
 Preventing invalid transactions

4. TYPES OF TRIGGERS
 1. Based on Timing (When they fire)
 BEFORE Trigger Executes before the
→
triggering statement
(INSERT/UPDATE/DELETE).
 AFTER Trigger Executes after the
→
triggering statement.
 INSTEAD OF Trigger Used for
→ views
(cannot directly insert/update/delete on
views).

5. TRIGGERS
 2. Based on Events
 DML Triggers Fire on INSERT, UPDATE,
→
DELETE.
 DDL Triggers Fire on CREATE, ALTER, DROP
→
(schema-level events).
 Database Triggers Fire on events like
→
LOGON, LOGOFF, STARTUP, SHUTDOWN.

6. TRIGGERS
 3. Based on Level
 Row-level trigger Executes once for each
→
row affected. (FOR EACH ROW)
 Statement-level trigger Executes once
→
per statement, regardless of row count.

7. SYNTAX FOR TRIGGERS
CREATE [OR REPLACE] TRIGGER trigger_name
{BEFORE | AFTER | INSTEAD OF}
{INSERT [OR] | UPDATE [OR] | DELETE}
ON table_name
[FOR EACH ROW] -- optional (row-level trigger)
[WHEN (condition)] -- optional
DECLARE
-- variable declarations
BEGIN
-- trigger body
END;
/

8. DML TRIGGERS
 The Data Definition Language (DDL)
command events such as Create_table,
Create_view, drop_table, Drop_view, and
Alter_table cause the DDL triggers to be
activated.
 They allow us to track changes in the
structure of the database. The trigger will
prevent any table creation, alteration, or
deletion in the database.

9. EXAMPLE: PREVENT TABLE
DELETIONS
CREATE TRIGGER prevent_table_creation
ON DATABASE
FOR CREATE_TABLE, ALTER_TABLE,
DROP_TABLE
AS
BEGIN
PRINT 'you can not create, drop and alter
table in this database';
ROLLBACK;
END;

10. OUTPUT:

11. DML TRIGGERS
 DML triggers fire when we manipulate data
with commands like INSERT, UPDATE, or
DELETE.
 These triggers are perfect for scenarios
where we need to validate data before it is
inserted, log changes to a table, or cascade
updates across related tables.

12. EXAMPLE: PREVENT
UNAUTHORIZED UPDATES
 Let’s say you want to prevent users from
updating the data in a sensitive students table.
 We can set up a trigger to handle that.
CREATE TRIGGER prevent_update
ON students
FOR UPDATE, INSERT, DELETE
AS
BEGIN
RAISERROR ('You can not insert, update and
delete rows in this table.', 16, 1);
END;

13. OUTPUT: