Create A Trigger In Mysql

MySQL trigger syntax

Update one table based on another table. This can be achieved when changes are made by using triggers.

In order to create a new trigger, you use the CREATE TRIGGER statement. The following illustrates the syntax of the CREATE TRIGGER statement:

CREATE TRIGGER trigger_name trigger_time trigger_event
ON table_name
FOR EACH ROW
BEGIN
...
END;

Let’s examine the syntax above in more detail.

• You put the trigger name after the CREATE TRIGGER statement. The trigger name should follow the naming convention [trigger time]_[table name]_[trigger event], for example before_employees_update.
• Trigger activation time can be BEFORE or AFTER. You must specify the activation time when you define a trigger. You use the BEFORE keyword if you want to process action prior to the change is made on the table and AFTER if you need to process action after the change is made.
• The trigger event can be INSERT, UPDATE or DELETE. This event causes the trigger to be invoked. A trigger only can be invoked by one event. To define a trigger that is invoked by multiple events, you have to define multiple triggers, one for each event.
• A trigger must be associated with a specific table. Without a table trigger would not exist therefore you have to specify the table name after the ON keyword.
• You place the SQL statements between BEGIN and END block. This is where you define the logic for the trigger.

MySQL trigger example

Let’s start creating a trigger in MySQL to log the changes of the employees table.

First, create a new table named employees_audit to keep the changes of the employee table. The following statement creates the employee_audit table.

CREATE TABLE employees_audit (
id INT AUTO_INCREMENT PRIMARY KEY,
employeeNumber INT NOT NULL,
lastname VARCHAR(50) NOT NULL,
changedat DATETIME DEFAULT NULL,
action VARCHAR(50) DEFAULT NULL
);

Next, create a BEFORE UPDATE trigger that is invoked before a change is made to the employees table.

DELIMITER $$
CREATE TRIGGER before_employee_update
BEFORE UPDATE ON employees
FOR EACH ROW
BEGIN
INSERT INTO employees_audit
SET action = 'update',
employeeNumber = OLD.employeeNumber,
lastname = OLD.lastname,
changedat = NOW();
END$$
DELIMITER ;

Inside the body of the trigger, we used the OLD keyword to access employeeNumber and lastnamecolumn of the row affected by the trigger.

Notice that in a trigger defined for INSERT, you can use NEW keyword only. You cannot use the OLD keyword. However, in the trigger defined for DELETE, there is no new row so you can use the OLDkeyword only. In the UPDATE trigger, OLD refers to the row before it is updated and NEW refers to the row after it is updated.

Then, to view all triggers in the current database, you use SHOW TRIGGERS statement as follows:

SHOW TRIGGERS;

In addition, if you look at the schema using MySQL Workbench under the employees > triggers, you will see the before_employee_update trigger

After that, update the employees table to check whether the trigger is invoked

UPDATE employees
SET
lastName = 'Phan'
WHERE
employeeNumber = 1056;

Finally, to check if the trigger was invoked by the UPDATE statement, you can query the employees_audit table using the following query

SELECT
*
FROM
employees_audit;

You will see the trigger was invoked as it has inserted a new row into the employees_audit table.

Let’s say you only wish to insert a new row into employees_audit when a particular column has changed value. In the example below a new row will only be inserted if the employeeNumber has changed.

DELIMITER $$
CREATE TRIGGER before_employee_update
    BEFORE UPDATE ON employees
    FOR EACH ROW
    BEGIN
        IF NEW.employeeNumber <> OLD.employeeNumber THEN
            INSERT INTO employees_audit
            SET action = 'update',
            employeeNumber = OLD.employeeNumber,
            lastname = OLD.lastname,
            changedat = NOW();
        END IF;
    END$$

In this tutorial, you have learned how to create a trigger in MySQL. We also showed you how to develop a trigger to audit the changes of the employees table.

A Full Example

 DELIMITER $$

CREATE TRIGGER table1_bi BEFORE INSERT ON table1 FOR EACH ROW
BEGIN
  INSERT INTO table2
	(ID, ColABc, Col2, Col3)
		VALUES
	(NEW.ID, NEW.ColABc, NEW.Col2, NEW.Col3);
END $$

CREATE TRIGGER table1_bu BEFORE UPDATE ON table1 FOR EACH ROW
BEGIN
  UPDATE table2
	SET
	ID = NEW.ID,
	ColABc = NEW.ColABc,
	Col2 = NEW.Col2,
	Col3 = NEW.Col3
   WHERE ID = OLD.ID;
END $$

CREATE TRIGGER table1_bd BEFORE DELETE ON table1 FOR EACH ROW
BEGIN
   DELETE FROM table2
   WHERE ID= OLD.ID;
END $$

DELIMITER ;