What is a trigger?

A trigger is a special stored procedure in a database that is automatically executed when a specific event occurs.

Events that invoke triggers typically include:

• INSERT: adding new rows to a table.
• UPDATE: modifying existing rows in a table.
• DELETE: removing rows from a table.

Triggers can be invoked at different times relative to the event:

• BEFORE: before the event is executed.
• AFTER: after the event is executed.
• INSTEAD OF: instead of executing the event (often used with views).

Examples of trigger usage:

1. Maintaining data integrity: Automatic checking or modification of data during insert/update.
2. Audit logging: Recording information about data changes in a separate log table.
3. Data synchronization: Automatic updating of related tables.
4. Executing complex business rules: Initiating additional actions under certain conditions.

Triggers are defined at the table level and can reference old and new values of the row that was changed.

-- Example syntax (SQL)
CREATE TRIGGER audit_log
AFTER INSERT ON users
FOR EACH ROW
BEGIN
    INSERT INTO audit (user_id, action, timestamp)
    VALUES (NEW.id, 'insert', NOW());
END;

It is important to use triggers with caution, as they can complicate debugging and affect database performance.