SQL - Triggers and Event-Driven Automation in SQL

1. What is a Trigger?

A trigger is a special type of stored program in SQL that automatically runs when a specified event occurs in a database table.
You do not execute triggers manually. They are activated by actions such as inserting, updating, or deleting data.

In simple terms, a trigger performs automatic tasks when certain database events happen.

2. Why Use Triggers?

  • Enforce business rules

  • Maintain data consistency

  • Automatically update related tables

  • Track changes or create logs

  • Validate data before storing it

Triggers help automate repetitive or important actions without requiring manual intervention.

3. Types of Trigger Events

INSERT Trigger
Runs when new data is added.

UPDATE Trigger
Runs when existing data is modified.

DELETE Trigger
Runs when data is removed.

These triggers respond to changes made in tables.

4. Timing of Triggers

BEFORE Trigger
Executes before the event occurs.
Useful for validation or modification.

AFTER Trigger
Executes after the event completes.
Useful for logging or follow-up actions.

Example concept:

  • BEFORE INSERT to check data validity

  • AFTER DELETE to record deletion history

5. Basic Syntax Example

CREATE TRIGGER trigger_name AFTER INSERT ON employees FOR EACH ROW BEGIN INSERT INTO log_table VALUES ('New employee added'); END;

This example records an entry when a new employee is inserted.

6. Event-Driven Automation

Triggers support event-driven automation, meaning actions occur automatically based on database events.

Examples:

  • Updating inventory after a sale

  • Recording transaction history

  • Sending alerts or notifications

  • Maintaining audit trails

This reduces manual coding and ensures consistent behavior.

7. Advantages and Considerations

Advantages

  • Automatic execution

  • Improves consistency

  • Reduces repetitive tasks

  • Helps enforce rules

Considerations

  • Overuse can affect performance

  • Harder to debug than regular queries

  • Complex logic may reduce readability

Summary

Triggers enable automatic execution of SQL actions when specified database events occur. They play an important role in event-driven automation by enforcing rules, maintaining consistency, and logging changes. Understanding triggers helps in building responsive and reliable database systems.