How can we create multiple MySQL triggers for the same trigger event and action time?

MySQL 5.7.2+ allows us to create multiple triggers for the same event and action time in a table. Both the triggers will activate sequentially when the event occurs. It can be understood with the help of an example −

Example

In this example, we are creating multiple triggers for the same event say BEFORE UPDATE. The names of the triggers are ‘Studentdetail_before_update’ and ‘Studentdetail_before_update2’. They will activate sequentially when an event occurs. We are creating these triggers on the table ‘Student_detail’ having the following data −

mysql> Select * from Student_detail;
+-----------+-------------+------------+
| Studentid | StudentName | address    |
+-----------+-------------+------------+
| 100       | Gaurav      | Delhi      |
| 101       | Raman       | Shimla     |
| 103       | Rahul       | Jaipur     |
| 104       | Ram         | Chandigarh |
| 105       | Mohan       | Chandigarh |
+-----------+-------------+------------+
5 rows in set (0.06 sec)

mysql> Delimiter //

Now with the help of the following query, we will create the first trigger, which will be created by the same query as earlier.

mysql> Create Trigger studentdetail_before_update
    -> BEFORE UPDATE
    -> ON Student_detail
    -> FOR EACH ROW
    -> BEGIN
    -> DECLARE AUSER Varchar(40);
    -> SELECT USER() into AUSER;
    ->INSERT INTO Student_detail_updated(studentid, Updated_date,Updated_by) values(OLD.studentid,NOW(),AUSER);
    -> END; //
Query OK, 0 rows affected (0.17 sec)

mysql> Update student_detail SET Address = 'Ludhiana' Where studentName = 'Ram';
Query OK, 1 row affected (0.15 sec)
Rows matched: 1 Changed: 1 Warnings: 0

After invoking the above-created trigger we got the following result −

mysql> Select * from student_detail_updated;
+-----------+---------------------+----------------+
| studentid | Updated_date        | Updated_by     |
+-----------+---------------------+----------------+
| 104       | 2017-11-22 16:17:16 | root@localhost |
+-----------+---------------------+----------------+
1 row in set (0.00 sec)

Now, the second trigger of the same event and action time can be created as follows −

mysql> Create Trigger studentdetail_before_update2
    -> BEFORE UPDATE
    -> ON Student_detail
    -> FOR EACH ROW FOLLOWS studentdetail_before_update
    -> BEGIN
    -> DECLARE AUSER Varchar(40);
    -> SELECT USER() into AUSER;
    -> INSERT INTO Student_detail_updated(studentid, Updated_date,Updated_by) values(OLD.studentid,NOW(),AUSER);
    -> END; //
Query OK, 0 rows affected (0.15 sec)

The above trigger will activate after the first trigger because we are using the keyword ‘FOLLOWS’.

mysql> Update Student_detail SET Address = 'Patiala' WHERE studentname = 'Mohan';
Query OK, 1 row affected (0.08 sec)
Rows matched: 1 Changed: 1 Warnings: 0

Now, when we update the value, the following result set is showing two rows for the same event and action time. Second row represents the value after studentdetail_before_update trigger and third row represents the value after studentdetail_before_update2 trigger.

mysql> Select * from student_detail_updated;
+-----------+---------------------+----------------+
| studentid | Updated_date        | Updated_by     |
+-----------+---------------------+----------------+
| 104       | 2017-11-22 16:17:16 | root@localhost |
| 105       | 2017-11-22 16:19:28 | root@localhost |
| 105       | 2017-11-22 16:19:28 | root@localhost |
+-----------+---------------------+----------------+
3 rows in set (0.00 sec)