MySQL SHOW Triggers

Introduction

In this chapter, we will learn how to use the SHOW TRIGGERS statement in MySQL. The SHOW TRIGGERS statement is used to list all the triggers associated with a specific database. This is particularly useful for managing and maintaining triggers, as it provides a quick overview of all the triggers and their details. We will cover the syntax for the SHOW TRIGGERS statement, examples of its usage, and important considerations for using it in MySQL.

Syntax

The basic syntax for the SHOW TRIGGERS statement is:

SHOW TRIGGERS [FROM db_name] [LIKE 'pattern'];

• db_name: The name of the database from which to show the triggers. If not specified, the current database is used.
• pattern: A pattern to match the trigger names. If specified, only triggers whose names match the pattern are shown.

Example

Here is an example of how to use the SHOW TRIGGERS statement to list all triggers in the current database:

SHOW TRIGGERS;

Example with Database Name

If you want to list triggers from a specific database, use the FROM clause:

SHOW TRIGGERS FROM company;

Example with Pattern

You can use the LIKE clause to filter triggers by name:

SHOW TRIGGERS LIKE 'before%';

Full Example

Let’s go through a complete example where we create a database, tables, and triggers, and then use the SHOW TRIGGERS statement to list the triggers.

1. Create a Database

CREATE DATABASE company;

2. Select the Database

USE company;

3. Create the Employees and Audit Tables

CREATE TABLE employees (
    employee_id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL,
    department_id INT,
    salary DECIMAL(10, 2)
);

CREATE TABLE employee_audit (
    audit_id INT PRIMARY KEY AUTO_INCREMENT,
    employee_id INT,
    action VARCHAR(10),
    action_time DATETIME
);

4. Create Triggers

Before Update Trigger

CREATE TRIGGER before_employee_update
BEFORE UPDATE ON employees FOR EACH ROW
BEGIN
    INSERT INTO employee_audit (employee_id, action, action_time)
    VALUES (OLD.employee_id, 'update', NOW());
END;

After Insert Trigger

CREATE TRIGGER after_employee_insert
AFTER INSERT ON employees FOR EACH ROW
BEGIN
    INSERT INTO employee_audit (employee_id, action, action_time)
    VALUES (NEW.employee_id, 'insert', NOW());
END;

5. List All Triggers in the Current Database

SHOW TRIGGERS;

Important Considerations

• Permissions: Ensure you have the appropriate permissions to view the triggers in the database. Typically, you need the SHOW TRIGGERS privilege.
• Database Selection: If you do not specify a database name, the SHOW TRIGGERS statement will list triggers from the current database.
• Pattern Matching: Use the LIKE clause to filter triggers by name, which is useful for finding specific triggers among many.

Conclusion

The SHOW TRIGGERS statement in MySQL is used for listing and managing triggers in a database. This chapter covered the syntax for the SHOW TRIGGERS statement, provided examples of its usage, and discussed important considerations.