Trigger in DBMS

A Trigger is a special type of stored procedure that automatically executes when a specified event occurs in a database table.

Components of Trigger

1. Event

The operation that activates the trigger.

Examples:

• INSERT
• UPDATE
• DELETE

2. Condition

A query/check evaluated when the trigger is activated.

If the condition is TRUE, the trigger executes.

3. Action

The procedure/statements executed when the condition is satisfied.

Formula

Trigger = Event + Condition + Action

Working of Trigger

1. An event occurs on a table
2. Condition is checked
3. If condition is TRUE → action executes automatically

Basic Syntax

CREATE TRIGGER trigger_name
BEFORE | AFTER INSERT | UPDATE | DELETE
ON table_name
FOR EACH ROW
WHEN (condition)
BEGIN
    action;
END;

Example

CREATE TRIGGER check_salary
BEFORE INSERT
ON employee
FOR EACH ROW
WHEN (NEW.salary < 10000)
BEGIN
    UPDATE employee
    SET salary = 10000;
END;

Types of Triggers

Based on Timing

• BEFORE Trigger
• AFTER Trigger

Based on Level

• Row Level Trigger
• Statement Level Trigger

Uses of Trigger

• Automatic validation
• Auditing/logging
• Maintaining integrity
• Security enforcement
• Automatic updates