Trigger

Introduction to SQL Trigger


Summary: in this tutorial, we will  give you a brief overview of SQL trigger, its advantages and disadvantages.

A SQL trigger is a set of  SQL statements stored in the database catalog. A SQL trigger is executed or fired whenever an event associated with a table occurs e.g.,  insert, update or delete.

A SQL trigger is a special type of stored procedure. It is special because it is not called directly like a stored procedure. The main difference between a trigger and a stored procedure is that a trigger is called automatically when a data modification event is made against a table whereas a stored procedure must be called explicitly.

It is important to understand SQL trigger’s advantages and disadvantages so that you can use it appropriately. In the following sections, we will discuss about the advantages and disadvantages of using SQL triggers.

Advantages of using SQL triggers

SQL triggers provide an alternative way to check the integrity of data.
SQL triggers can catch errors in business logic in the database layer.
SQL triggers provide an alternative way to run scheduled tasks. By using SQL triggers, you don’t have to wait to run the scheduled tasks because the triggers are invoked  automatically before or after a change  is made to the data in tables.
SQL triggers are very useful to audit the changes of data in tables.
Disadvantages of using SQL triggers

SQL triggers only can provide an extended validation and they cannot replace all the validations. Some simple validations have to be done in the application layer. For example, you can validate user’s inputs in the client side by using JavaScript or in the server side using server side scripting languages such as JSP, PHP, ASP.NET, Perl, etc.
SQL triggers are invoked and executed invisibly from client-applications therefore it is difficult to figure out what happen in the database layer.
SQL triggers may increase the overhead of the database server.

CREATE TRIGGER Syntax

CREATE
    [DEFINER = { user | CURRENT_USER }]
    TRIGGER trigger_name trigger_time trigger_event
    ON tbl_name FOR EACH ROW trigger_stmt

DROP TRIGGER Syntax

DROP TRIGGER [IF EXISTS] [schema_name.]trigger_name


Example