Introduction to TRIGGERS IN SQL

A trigger is a special kind of stored procedure that automatically executes when an event occurs in the database server. 

DML triggers execute when a user tries to modify data through a data manipulation language (DML) event. DML events are INSERT, UPDATE, or DELETE statements on a table or view. These triggers fire when any valid event is fired, regardless of whether or not any table rows are affected. 

Unlike stored procedures, an SQL trigger cannot be directly called from an application. Instead, an SQL trigger is invoked by the database management system on the execution of a triggering insert, update, or delete operation. The definition of the SQL trigger is stored in the database management system and is invoked by the database management system, when the SQL table, that the trigger is defined on, is modified.

An SQL trigger may call stored procedures or user-defined functions to perform additional processing when the trigger is executed.

Advantages of using SQL trigger

§                       SQL Trigger provides an alternative way to check integrity.

§                       SQL trigger can catch the errors in business logic in the database level.

§                       SQL trigger is very useful when you use it to audit the changes of data in a database table.

Syntax of Triggers

The Syntax for creating a trigger is:

 CREATE [OR REPLACE ] TRIGGER trigger_name 

 {BEFORE | AFTER | INSTEAD OF } 

 {INSERT [OR] | UPDATE [OR] | DELETE} 

 [OF col_name] 

 ON table_name 

 [REFERENCING OLD AS o NEW AS n] 

 [FOR EACH ROW] 

 WHEN (condition)  

 BEGIN 

   --- sql statements  

 END; 

• CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or overwrites an existing trigger with the same name.
• {BEFORE | AFTER | INSTEAD OF } - This clause indicates at what time should the trigger get fired. i.e for example: before or after updating a table. INSTEAD OF is used to create a trigger on a view. before and after cannot be used to create a trigger on a view.
• {INSERT [OR] | UPDATE [OR] | DELETE} - This clause determines the triggering event. More than one triggering events can be used together separated by OR keyword. The trigger gets fired at all the specified triggering event.
• [OF col_name] - This clause is used with update triggers. This clause is used when you want to trigger an event only when a specific column is updated.
• CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or overwrites an existing trigger with the same name.
• [ON table_name] - This clause identifies the name of the table or view to which the trigger is associated.
• [REFERENCING OLD AS o NEW AS n] - This clause is used to reference the old and new values of the data being changed. By default, you reference the values as :old.column_name or :new.column_name. The reference names can also be changed from old (or new) to any other user-defined name. You cannot reference old values when inserting a record, or new values when deleting a record, because they do not exist.
• [FOR EACH ROW] - This clause is used to determine whether a trigger must fire when each row gets affected ( i.e. a Row Level Trigger) or just once when the entire sql statement is executed(i.e.statement level Trigger).
• WHEN (condition) - This clause is valid only for row level triggers. The trigger is fired only for rows that satisfy the condition specified.