Faculty notes You need to provide each trigger with a name and define it for a particular table or view in a database. You also have an option of encrypting the trigger so that no one (not even the owner) can look at the original code. Triggers can be used to enforce referential integrity: You could create a trigger that, upon the insertion of a record looks for the corresponding value of the foreign key in the parent table. If the value isn’t found, the transaction can be rolled back. Difference between triggers and declarative constraints A declarative integrity constraint applies to the data which is existing in the table and any statement that manipulates the table. A trigger does not apply to the data which was there in the table before the trigger was created. The triggers are used to implement more complex data integrity. The Triggers except InsteadOfTriggers are Reactive. But the declarative constraints are Proactive. When the table is manipulated by using Insert, Update and Delete statements, first the table is updated and the trigger gets executed. But the declarative constraints are checked first and the table is manipulated.
Faculty Notes: Setting the triggering order: In SQL Server 7.0, triggers were understood to be fired in no particular order. Since SQL Server 2000, a new stored procedure has been added called sp_settriggerorder. The purpose of sp_settriggerorder is to set for a particular operation (INSERT, UPDATE, or DELETE) for a particular table which trigger will fire first and/or which trigger will fire last. Any and all triggers not specified as first or last will fire in no particular order. Our stored procedure has the following syntax: EXEC sp_settriggerorder <trigger name> , <order> , ' <operation> ‘ For instance: EXEC sp_settriggerorder trig_insert_Employee, first, 'INSERT‘ or EXEC sp_settriggerorder @triggername=‘myTrigger’, @order = ‘first’ We have three choices on order: first, last, and none. We can use none to toggle a trigger to no longer fire first or last.
Faculty Notes: After you have specified which table or view is to be the beneficiary of the trigger, you need to define the trigger as an AFTER or INSTEAD OF trigger. You cannot define an AFTER trigger for a view.
Faculty notes Explanation of the above example : When a new employee is added to the employee table, the no_of_emp column value in the department table for a particular department to which the employee is added, will be incremented by one. For the above example to execute, run the following script. Create the following 2 tables, Employee Table: Create table employee ( emp_code char(4), emp_name varchar(15), city varchar(15), age int, phone char(14), dept_code char(4), doj datetime, salary money, email varchar(20) ) Department table: Create table department ( dept_code char(4), dept_name varchar(15), no_of_emp int ) Execute the below statement to see whether the trigger executes. insert into employee values ('E002','Raghu','Bangalore',45, '(044)-23423434','D002','10-10-2005',150000, 'email@example.com', 'M') Result : check to see if the no_of_emp value of the department ‘D002’ in the department table is incremented by one.
Faculty notes When the above query is executed, The row is inserted into the employee table and the same row is inserted into the inserted table. As there are many rows in the employee table, we can get access to the row that is just inserted based on the row in the inserted table. And this row in the inserted table is available only during the execution of the trigger.
Faculty notes Use the scripts that are mentioned in the previous slide before you create the trigger mentioned in the above slide. Result : When the above delete statement is executed, check to see whether the value of the no_of_emp column is decremented by 1. for the department of the Employee ‘E002’.
Faculty notes We have executed the following query: delete from employee where id=‘E002’ When we delete a record from employee table, the deleted record would be put in the deleted table as shown in the above figure. The deleted table stores copies of the affected rows during DELETE and UPDATE statements. During the execution of a DELETE or UPDATE statement, rows are deleted from the trigger table and moved to the deleted table. The deleted table and the trigger table have no rows in common. In the above figure, employee is acting as a trigger table.
Faculty notes When, the below sql Statement is executed, update employee set dept_code = 'D003' where emp_code = 'E001‘ Assuming that the department code of employee ‘E001’ is ‘D001’, when the above update statement is executed, the no_of_emp of the dept_code column for D001 and D003 departments in the department table are decremented and incremented by 1 respectively. Also, initially it checks to see whether the dept_code is getting updated using the statement update(dept_code), by doing so the trigger body gets executed only if the dept_code is updated.
Faculty notes Updation is deletion followed by insertion. In the above example, the record is deleted first from the Employee and put into the deleted table, then the record is updated and inserted into to the Employee Table. After inserting the updated record into the Employee table, the record is inserted into the Inserted Table.
Faculty notes To turn off nested triggers, use: Exec sp_configure ‘nested triggers’,0 reconfigure To turn on nested triggers, use: Exec sp_configure ‘nested triggers’,1,reconfigure
Faculty notes Example of Indirect recursion: An update on table A fires a trigger which causes an update on table B. The update on table B fires a trigger which does an update on table C. Table C has a trigger which causes an update on table A again. Table A’s trigger fires again. Recursive triggers create a lot of problems. As a result, recursive triggers are turned off by default. If you want to check the status of recursive triggers in a particular database, use: Exec sp_dboption ‘<name of database>’,’recursive triggers’ To turn on recursive triggers, use: Exec db_option ‘<name of database>’, ’recursive triggers’, ’true’ To turn off recursive triggers, use: Exec db_option ‘<name of database>’,’recursive triggers’, ‘false’
The below trigger gets executed when a new employee is added to the employee table.
create trigger trgInsertEmp on employee for insert as begin declare @dcode char(4) select @dcode = dept_code from inserted update department set no_of_emp = no_of_emp + 1 where dept_code = @dcode print 'The Department updated succesfully' end