Triggers Concept

A Trigger is a named database object which defines some action that the database should take when some databases related event occurs. Triggers are executed when you issues a data manipulation command like INSERT, DELETE, UPDATE on a table for which the trigger has been created. They are automatically executed and also transparent to the user. But for creating the trigger the user must have the CREATE TRIGGER privilege. In this section we will describe you about the syntax to create and drop the triggers and describe you some examples of how to use them.

CREATE TRIGGER

The general syntax of CREATE TRIGGER is :
        CREATE TRIGGER trigger_name trigger_time trigger_event ON tbl_name FOR EACH ROW trigger_statement

By using above statement we can create the new trigger. The trigger can associate only with the table name and that must be refer to a permanent table. Trigger_time means trigger action time. It can be BEFORE or AFTER. It is used to define that the trigger fires before or after the statement that executed it. Trigger_event specifies the statement that executes the trigger. The trigger_event can be any of the DML Statement : INSERT, UPDATE, DELETE.

We can not have the two trigger for a given table, which have the same trigger action time and event. For Instance : we cannot have two BEFORE INSERT triggers for same table. But we can have a BEFORE INSERT and BEFORE UPDATE trigger for a same table

Trigger_statement have the statement that executes when the trigger fires but if you want to execute multiple statement the you have to use the BEGIN…END compound statement.

We can refer the columns of the table that associated with trigger by using the OLD and NEW keyword. OLD.column_name is used to refer the column of an existing row before it is deleted or updated and NEW.column_name is used to refer the column of a new row that is inserted or after updated existing row.

In INSERT trigger we can use only NEW.column_name because there is no old row and in a DELETE trigger we can use only OLD.column_name because there is no new row. But in UPDATE trigger we can use both, OLD.column_name is used to refer the columns of a row before it is updated and NEW.Column_name is used to refer the column of the row after it is updated.

In the following example we are updating the Salary column of Employee table before inserting any record in Emp table. Example :

mysql> SELECT * FROM Employee;
+-----+---------+----------+-------------------+--------+-------+
| Eid | Ename   | City     | Designation       | Salary | Perks |
+-----+---------+----------+-------------------+--------+-------+
| 1   | Rahul   | Delhi    | Manager           | 10300  | 853   |
| 2   | Gaurav  | Mumbai   | Assistant Manager | 10300  | 853   |
| 3   | Chandan | Banglore | Team Leader       | 15450  | 999   |
| 5   | Tapan   | Pune     | Developer         | 20600  | 1111  |
| 6   | Amar    | Chennai  | Developer         | 16000  | 1124  |
| 7   | Santosh | Delhi    | Designer          | 10000  | 865   |
| 8   | Suman   | Pune     | Web Designer      | 20000  | 658   |
+-----+---------+----------+-------------------+--------+-------+
7 rows in set (0.00 sec)

mysql> delimiter //
mysql> CREATE TRIGGER ins_trig BEFORE INSERT ON Emp
    -> FOR EACH ROW
    -> BEGIN
    -> UPDATE Employee SET Salary=Salary-300 WHERE Perks>500;
    -> END;
    -> //
Query OK, 0 rows affected (0.01 sec)
mysql> delimiter ;
mysql> INSERT INTO Emp VALUES(9,'Rajesh','Delhi','Developer',15000,658);
Query OK, 1 row affected (0.05 sec)
mysql> SELECT * FROM Employee;
+-----+---------+----------+-------------------+--------+-------+
| Eid | Ename   | City     | Designation       | Salary | Perks |
+-----+---------+----------+-------------------+--------+-------+
| 1   | Rahul   | Delhi    | Manager           | 10000  | 853   |
| 2   | Gaurav  | Mumbai   | Assistant Manager | 10000  | 853   |
| 3   | Chandan | Banglore | Team Leader       | 15150  | 999   |
| 5   | Tapan   | Pune     | Developer         | 20300  | 1111  |
| 6   | Amar    | Chennai  | Developer         | 15700  | 1124  |
| 7   | Santosh | Delhi    | Designer          | 9700   | 865   |
| 8   | Suman   | Pune     | Web Designer      | 19700  | 658   |
+-----+---------+----------+-------------------+--------+-------+
7 rows in set (0.00 sec)
 
In the following example we are modifying the salary of Employee table before updating the record of the same table. Example :



mysql> delimiter //
mysql> CREATE TRIGGER updtrigger BEFORE UPDATE ON Employee
    -> FOR EACH ROW
    -> BEGIN
    -> IF NEW.Salary<=500 THEN
    -> SET NEW.Salary=10000;
    -> ELSEIF NEW.Salary>500 THEN
    -> SET NEW.Salary=15000;
    -> END IF;
    -> END
    -> //
Query OK, 0 rows affected (0.01 sec)
mysql> delimiter ;
mysql> UPDATE Employee
    -> SET Salary=500;
Query OK, 5 rows affected (0.04 sec)
Rows matched: 7  Changed: 5  Warnings: 0
mysql> SELECT * FROM Employee;
+-----+---------+----------+-------------------+--------+-------+
| Eid | Ename   | City     | Designation       | Salary | Perks |
+-----+---------+----------+-------------------+--------+-------+
| 1   | Rahul   | Delhi    | Manager           | 10000  | 853   |
| 2   | Gaurav  | Mumbai   | Assistant Manager | 10000  | 853   |
| 3   | Chandan | Banglore | Team Leader       | 10000  | 999   |
| 5   | Tapan   | Pune     | Developer         | 10000  | 1111  |
| 6   | Amar    | Chennai  | Developer         | 10000  | 1124  |
| 7   | Santosh | Delhi    | Designer          | 10000  | 865   |
| 8   | Suman   | Pune     | Web Designer      | 10000  | 658   |
+-----+---------+----------+-------------------+--------+-------+
7 rows in set (0.00 sec)
mysql> UPDATE Employee
    -> SET Salary=1500;
Query OK, 7 rows affected (0.03 sec)
Rows matched: 7  Changed: 7  Warnings: 0
mysql> SELECT * FROM Employee;
+-----+---------+----------+-------------------+--------+-------+
| Eid | Ename   | City     | Designation       | Salary | Perks |
+-----+---------+----------+-------------------+--------+-------+
| 1   | Rahul   | Delhi    | Manager           | 15000  | 853   |
| 2   | Gaurav  | Mumbai   | Assistant Manager | 15000  | 853   |
| 3   | Chandan | Banglore | Team Leader       | 15000  | 999   |
| 5   | Tapan   | Pune     | Developer         | 15000  | 1111  |
| 6   | Amar    | Chennai  | Developer         | 15000  | 1124  |
| 7   | Santosh | Delhi    | Designer          | 15000  | 865   |
| 8   | Suman   | Pune     | Web Designer      | 15000  | 658   |
+-----+---------+----------+-------------------+--------+-------+
7 rows in set (0.01 sec)
 
DROP TRIGGER

The general syntax of DROP TRIGGER is :
        DROP TRIGGER trigger_name

This statement is used to drop a trigger. Example of Dropping the Trigger : 



mysql> DROP TRIGGER updtrigger;
Query OK, 0 rows affected (0.02 sec)