TRIGGERS
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)
|
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Current Comments
8 comments so far (post your own) View All Comments Latest 10 Comments:i want to use trigger for updating a field when other fields are updated but when not that particular field is updated
Posted by darima on Sunday, 05.11.08 @ 14:15pm | #59330
Hi there
Can you make me a table that get top number of customers who spend more money on buying products. If example there is a product and customer tables.
Can you make me triger that get top number of stores where got the most revenue from selling the most products. if example there is a store table.
Please
Cheers
Posted by TOm on Saturday, 05.10.08 @ 21:41pm | #59190
night sir?
i have any question,
how can i make trigger database?
this is will i use to built sms gateway autoreply..
please reply to my mail.
thanks!
regards
mapays-semarang,central java
Posted by mapays on Wednesday, 04.2.08 @ 19:23pm | #55134
I want more explanation with suitable example about Delete and Insert Triggers.
Posted by A.D. Reddy on Saturday, 11.10.07 @ 14:25pm | #36837
hi
can any one explain me about trigger clearly
Posted by karthika on Sunday, 11.4.07 @ 19:48pm | #35522
This trigger chapater is very nice .it cleared my doughts
Thank you.
Posted by lavanya on Saturday, 10.13.07 @ 14:32pm | #33477
i want create trigger for user can nor delete 1 row which this row is default row .
when this row was deleted trigger permission not allow.
Posted by Ali Ramezani on Monday, 07.2.07 @ 13:27pm | #20580
How to update record by one trigger?
Posted by Patel Sanjay J on Thursday, 04.19.07 @ 10:30am | #14536