MySQL Triggers: Complete Usage Guide

A Trigger in MySQL is a special database object that triggers specific actions on a specified database table, such as insert, update, delete, etc. Triggers can automatically perform corresponding actions when data changes, which can be SQL statements, stored procedures, or custom functions.

The main aspects of Trigger usage include the following:

1. Creating Triggers: Use the CREATE TRIGGER statement to create a trigger, specifying the event (INSERT, UPDATE, DELETE), timing (BEFORE or AFTER), and action triggered (SQL statement, stored procedure, or custom function).
2. Events that trigger actions: Triggers can perform corresponding operations when INSERT, UPDATE, or DELETE events occur on a database table. By specifying the BEFORE or AFTER keyword, you can control whether the trigger executes before or after the event.
3. Timing of Trigger: Triggers can be executed before (BEFORE) or after (AFTER) an event occurs. A BEFORE Trigger can modify or validate data before it is inserted, updated, or deleted, while an AFTER Trigger can perform other operations after data has been inserted, updated, or deleted.
4. Operations triggered by a trigger can perform various actions, such as executing SQL statements, calling stored procedures, or custom functions. The NEW and OLD keywords can be used in triggers to reference new data about to be inserted, updated, or deleted, and the old data.
5. Activation of Triggers: Triggers are associated with database tables, so they will automatically be activated and executed when the corresponding events occur on the database table.
6. Delete Trigger: You can remove a trigger by using the DROP TRIGGER statement.

In conclusion, Triggers in MySQL provide a mechanism for automatically performing operations, triggering corresponding actions when specific events occur on database tables, enhancing the flexibility and functionality of the database.