Detailed explanation of triggers in MySQL

MySQL is a popular relational database management system that can implement automated business logic and operations through triggers. A trigger is a stored procedure that is automatically executed when a specific operation occurs on a database table and can respond to specific events, such as INSERT, UPDATE, and DELETE statements.

This article will introduce in detail the concept, creation and use of triggers in MySQL, as well as some precautions.

1. Concept

A trigger is a piece of code associated with a table that automatically executes when a specific event (INSERT, UPDATE, and DELETE statements) occurs. Triggers can automatically execute stored procedures when data is inserted or updated in a data table to implement constraints, default values, or handle business logic.

Triggers can be created and deleted in MySQL, and the definition of an already created trigger can be modified. Each trigger has a trigger event and a response event. The trigger event is usually an INSERT, UPDATE, or DELETE statement on the data table. The response event is the operation performed by the MySQL server after the trigger event.

2. Create a trigger

In MySQL, creating a trigger requires the following steps:

1. Define the trigger name
The trigger name should be unique so that the corresponding trigger can be easily located.

2. Define events
Triggers are generally triggered based on INSERT, UPDATE or DELETE events.

3. Define the trigger time
The trigger time refers to when the trigger is triggered. MySQL provides two triggers:

BEFORE trigger: in the INSERT, UPDATE or DELETE statement Triggered before execution;

AFTER trigger: triggered after execution of INSERT, UPDATE or DELETE statement.

4. Define trigger operations
Triggers can handle a variety of operations, such as setting default values, displaying error messages, universal formats, etc.

5. Define the trigger code
The trigger code is a stored procedure that is executed. The key is to process the data read from the data table when the event is triggered.

For example, the following code:

CREATE TRIGGER before_delete_user

BEFORE DELETE ON user

FOR EACH ROW

BEGIN

IF OLD.status = 'disabled' THEN

    CALL throw_error('The user is disabled and cannot be deleted.');

END IF;

END;

The above code means that before deleting the records in the user table, execute a stored procedure to determine whether the deleted user has been disabled. If so, it will prompt that it cannot be deleted.

3. Precautions for using triggers

You should pay attention to the following when using triggers:

1. Multiple events with the same event and time cannot be created on the same table. Triggers;

2. Triggers must be enabled in MySQL to be used;

3. Syntax errors may cause the trigger to fail to be created successfully;

4. Triggers The executed code may output debugging information when needed to find the problem;

5. If the trigger is executed recursively multiple times, you need to set parameters involving conditions externally or manually close the trigger.

4. Summary

This article introduces in detail the concept, creation and use of triggers in MySQL, as well as precautions. Triggers are a very useful function that can help developers process business logic efficiently and improve the maintainability of the code. There are limitations and syntax errors when using triggers that you should be aware of.

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