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TRIGGERS:


Introduction


We can define a Trigger as "A Trigger is a Database object just like a stored procedure or we can say it is a special kind of Stored Procedure which fires when an event occurs in a database.".

Difference between Stored Procedure and trigger 


Triggers fire implicitly while Stored Procedures fire explicitly.

Type of Triggers


There are two types of Triggers:
  1. DDL Trigger
  2. DML trigger

DDL Triggers


They fire in response to DDL (Data Definition Language) command events that start with Create, Alter and Drop. Like Create_table, Create_view, drop_table, Drop_view and Alter_table.

Code of DDL Trigger
  1. create trigger saftey  
  2. on database  
  3. for  
  4. create_table,alter_table,drop_table  
  5. as  
  6. print'you can not create ,drop and alter table in this database'  
  7. rollback; 
When we create, alter or drop any table in a database then the following message appears:
DDl-trigers-in-sql.jpg

DML Triggers

They fire in response to DML (Data Manipulation Language) command events that start with with Insert, Update and Delete. Like insert_table, Update_view and Delete_table.
  1. create trigger deep  
  2. on emp  
  3. for  
  4. insert,update,delete  
  5. as  
  6. print'you can not insert,update and delete this table i'  
  7. rollback;
When we insert, update or delete in a table in a database then the following message appears:
dml-triggers-in-sql.jpg
DML triggers are two type
AFTER Triggers
AFTER triggers are executed after the action of an INSERT, UPDATE, or DELETE statement.
  1. create trigger insertt  
  2. on emp  
  3. after insert  
  4. as  
  5. begin  
  6. insert into empstatus values('active')  
  7. end   
aftertrigger-in-sql.jpg

INSTEAD Of Triggers
It will tell the database engine to execute the trigger instead of executing the statement. For example an insert trigger executes when an event occurs instead of the statement that would insert the values in the table . 
  1. CREATE TRIGGER instoftr  
  2. ON v11  
  3. INSTEAD OF INSERT  
  4. AS  
  5. BEGIN  
  6. INSERT INTO emp  
  7. SELECT I.id, I.names  
  8. FROM INSERTED I  
  9.    
  10. INSERT INTO emp1values  
  11. SELECT I.id1, I.name1  
  12. FROM INSERTED I  
  13. END   
When we insert data into a view by the following query then it inserts values in both tables :
  1. insert into v11 values(1,'d','dd')  
You can see both tables by the folowing query:
  1. select * from emp  
  2. select * from emp1values  
insteedofcorsor-in-sql.jpg

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