Triggers -- Sql Server

INTRODUCTION

TRIGGERS IN SQL SERVER

BACKGROUND

This article gives a brief introduction about Triggers in Sql Server 2000/2005.

What is a Trigger

A trigger is a special kind of a store procedure that executes in response to certain action on the table like insertion, deletion or updation of data. It is a database object which is bound to a table and is executed automatically. You can’t explicitly invoke triggers. The only way to do this is by performing the required action no the table that they are assigned to.

Types Of Triggers

There are three action query types that you use in SQL which are INSERT, UPDATE and DELETE. So, there are three types of triggers and hybrids that come from mixing and matching the events and timings that fire them.

Basically, triggers are classified into two main types:- 

(i) After Triggers (For Triggers) 
(ii) Instead Of Triggers 

(i) After Triggers

These triggers run after an insert, update or delete on a table. They are not supported for views. 
AFTER TRIGGERS can be classified further into three types as: 

(a) AFTER INSERT Trigger. 
(b) AFTER UPDATE Trigger. 
(c) AFTER DELETE Trigger. 

Let’s create After triggers. First of all, let’s create a table and insert some sample data. Then, on this table, I will be attaching several triggers. 

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CREATE TABLE Employee_Test
(
Emp_ID INT Identity,
Emp_name Varchar(100),
Emp_Sal Decimal (10,2)
)

INSERT INTO Employee_Test VALUES ('Anees',1000);
INSERT INTO Employee_Test VALUES ('Rick',1200);
INSERT INTO Employee_Test VALUES ('John',1100);
INSERT INTO Employee_Test VALUES ('Stephen',1300);
INSERT INTO Employee_Test VALUES ('Maria',1400);

I will be creating an AFTER INSERT TRIGGER which will insert the rows inserted into the table into another audit table. The main purpose of this audit table is to record the changes in the main table. This can be thought of as a generic audit trigger. 

Now, create the audit table as:-

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CREATE TABLE Employee_Test_Audit
(
Emp_ID int,
Emp_name varchar(100),
Emp_Sal decimal (10,2),
Audit_Action varchar(100),
Audit_Timestamp datetime
)

(a) AFTRE INSERT Trigger

This trigger is fired after an INSERT on the table. Let’s create the trigger as:-

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CREATE TRIGGER trgAfterInsert ON [dbo].[Employee_Test] 
FOR INSERT
AS
 declare @empid int;
 declare @empname varchar(100);
 declare @empsal decimal(10,2);
 declare @audit_action varchar(100);

 select @empid=i.Emp_ID from inserted i; 
 select @empname=i.Emp_Name from inserted i; 
 select @empsal=i.Emp_Sal from inserted i; 
 set @audit_action='Inserted Record -- After Insert Trigger.';

 insert into Employee_Test_Audit
           (Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp) 
 values(@empid,@empname,@empsal,@audit_action,getdate());

 PRINT 'AFTER INSERT trigger fired.'
GO

The CREATE TRIGGER statement is used to create the trigger. THE ON clause specifies the table name on which thetrigger is to be attached. The FOR INSERT specifies that this is an AFTER INSERT trigger. In place of FOR INSERT, AFTER INSERT can be used. Both of them mean the same. 
In the trigger body, table named inserted has been used. This table is a logical table and contains the row that has been inserted. I have selected the fields from the logical inserted table from the row that has been inserted into different variables, and finally inserted those values into the Audit table. 
To see the newly created trigger in action, lets insert a row into the main table as : 

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 insert into Employee_Test values('Chris',1500);

Now, a record has been inserted into the Employee_Test table. The AFTER INSERT trigger attached to this table has inserted the record into the Employee_Test_Audit as:-

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6   Chris  1500.00   Inserted Record -- After Insert Trigger. 2008-04-26 12:00:55.700

(b) AFTER UPDATE Trigger

This trigger is fired after an update on the table. Let’s create the trigger as:-

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CREATE TRIGGER trgAfterUpdate ON [dbo].[Employee_Test] 
FOR UPDATE
AS
 declare @empid int;
 declare @empname varchar(100);
 declare @empsal decimal(10,2);
 declare @audit_action varchar(100);

 select @empid=i.Emp_ID from inserted i; 
 select @empname=i.Emp_Name from inserted i; 
 select @empsal=i.Emp_Sal from inserted i; 
 
 if update(Emp_Name)
  set @audit_action='Updated Record -- After Update Trigger.';
 if update(Emp_Sal)
  set @audit_action='Updated Record -- After Update Trigger.';

 insert into Employee_Test_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp) 
 values(@empid,@empname,@empsal,@audit_action,getdate());

 PRINT 'AFTER UPDATE Trigger fired.'
GO

The AFTER UPDATE Trigger is created in which the updated record is inserted into the audit table. There is no logical table updated like the logical table inserted. We can obtain the updated value of a field from theupdate(column_name) function. In our trigger, we have used, if update(Emp_Name) to check if the column Emp_Name has been updated. We have similarly checked the column Emp_Sal for an update. 
Let’s update a record column and see what happens. 

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 update Employee_Test set Emp_Sal=1550 where Emp_ID=6

This inserts the row into the audit table as:- 

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6  Chris  1550.00  Updated Record -- After Update Trigger.   2008-04-26 12:38:11.843 

(c) AFTER DELETE Trigger

This trigger is fired after a delete on the table. Let’s create the trigger as:- 

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CREATE TRIGGER trgAfterDelete ON [dbo].[Employee_Test] 
AFTER DELETE
AS
 declare @empid int;
 declare @empname varchar(100);
 declare @empsal decimal(10,2);
 declare @audit_action varchar(100);

 select @empid=d.Emp_ID from deleted d; 
 select @empname=d.Emp_Name from deleted d; 
 select @empsal=d.Emp_Sal from deleted d; 
 set @audit_action='Deleted -- After Delete Trigger.';

 insert into Employee_Test_Audit
(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp) 
 values(@empid,@empname,@empsal,@audit_action,getdate());

 PRINT 'AFTER DELETE TRIGGER fired.'
GO

In this trigger, the deleted record’s data is picked from the logical deleted table and inserted into the audit table. 
Let’s fire a delete on the main table. 
A record has been inserted into the audit table as:- 

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 6  Chris 1550.00  Deleted -- After Delete Trigger.  2008-04-26 12:52:13.867 

All the triggers can be enabled/disabled on the table using the statement 

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ALTER TABLE Employee_Test {ENABLE|DISBALE} TRIGGER ALL 

Specific Triggers can be enabled or disabled as :- 

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ALTER TABLE Employee_Test DISABLE TRIGGER trgAfterDelete

This disables the After Delete Trigger named trgAfterDelete on the specified table. 

(ii) Instead Of Triggers

These can be used as an interceptor for anything that anyonr tried to do on our table or view. If you define an Instead Of trigger on a table for the Delete operation, they try to delete rows, and they will not actually get deleted (unless you issue another delete instruction from within the trigger)
INSTEAD OF TRIGGERS can be classified further into three types as:- 

(a) INSTEAD OF INSERT Trigger. 
(b) INSTEAD OF UPDATE Trigger. 
(c) INSTEAD OF DELETE Trigger. 

(a) Let’s create an Instead Of Delete Trigger as:-

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CREATE TRIGGER trgInsteadOfDelete ON [dbo].[Employee_Test] 
INSTEAD OF DELETE
AS
 declare @emp_id int;
 declare @emp_name varchar(100);
 declare @emp_sal int;
 
 select @emp_id=d.Emp_ID from deleted d;
 select @emp_name=d.Emp_Name from deleted d;
 select @emp_sal=d.Emp_Sal from deleted d;

 BEGIN
  if(@emp_sal>1200)
  begin
   RAISERROR('Cannot delete where salary > 1200',16,1);
   ROLLBACK;
  end
  else
  begin
   delete from Employee_Test where Emp_ID=@emp_id;
   COMMIT;
   insert into Employee_Test_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp)
   values(@emp_id,@emp_name,@emp_sal,'Deleted -- Instead Of Delete Trigger.',getdate());
   PRINT 'Record Deleted -- Instead Of Delete Trigger.'
  end
 END
GO

This trigger will prevent the deletion of records from the table where Emp_Sal > 1200. If such a record is deleted, the Instead Of Trigger will rollback the transaction, otherwise the transaction will be committed. 
Now, let’s try to delete a record with the Emp_Sal >1200 as:- 

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delete from Employee_Test where Emp_ID=4

This will print an error message as defined in the RAISE ERROR statement as:- 

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Server: Msg 50000, Level 16, State 1, Procedure trgInsteadOfDelete, Line 15
Cannot delete where salary > 1200 

And this record will not be deleted. 
In a similar way, you can code Instead of Insert and Instead Of Update triggers on your tables. 

CONCLUSION

In this article, I took a brief introduction of triggers, explained the various kinds of triggers – After Triggers and Instead Of Triggers along with their variants and explained how each of them works. I hope you will get a clear understanding about the Triggers in Sql Server and their usage.

Sql Server - How to write a Stored procedure in Sql server

Stored Procedure:- Stored Procedure In Sql server can be defined as the set of logically group of sqlstatement which are grouped to perform a specific task. There are many benefits of using a stored procedure. The main benefit of using a stored procedure is that it increases the performance of the database.The other benefits of using the Stored Procedure is given below:-

Benefits of using the Stored Procedure

1. One of the main benefit of using the Stored procedure is that it reduces the amount of information sent to the database server. It can become more important benefit when the bandwidth of the network is less. Since if we send the sql query (statement)which is executing in a loop to the server through network and the network get disconnected then the execution of the sql statement don't returns the expected results, if the sql query is not used between Transaction statement and rollback statement is not used.
2. Compilation step is required only once when the stored procedure is created. Then after it does not required recompilation before executing unless it is modified and re utilizes the same execution plan whereas the sql statements needs to be compiled every time whenever it is sent for execution even if we send the same sql statement every time.
3. It helps in re usability of the sql code because it can be used by multiple users and by multiple client since we needs to just call the stored procedure instead of writing the same sql statement every time. It helps in reduces the development time.
4. Stored procedure is helpful in enhancing the security since we can grant permission to the user for executing the Stored procedure instead of giving the permission on the tables used in the Storedprocedure.
5. Sometime it is useful to use the database for storing the business logic in the form of stored proceduresince it make it secure and if any change is needed in the business logic then we may only need to make changes in the stored procedure and not in the files contained on the web server.

How to write a Stored Procedure in SQL Server

Suppose there is a table called tbl_Students whose structure is given below:

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CREATE TABLE  tbl_Students

(
    [Studentid] [int] IDENTITY(1,1) NOT NULL,
    [Firstname] [nvarchar](200) NOT  NULL,
    [Lastname] [nvarchar](200)  NULL,
    [Email] [nvarchar](100)  NULL
)

Support we insert the following data into the above table:-

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Insert into tbl_Students (Firstname, lastname, Email)
 Values('Vivek', 'Johari', 'vivek@abc.com')

Insert into tbl_Students (Firstname, lastname, Email)
 Values('Pankaj', 'Kumar', 'pankaj@abc.com')

Insert into tbl_Students (Firstname, lastname, Email)
 Values('Amit', 'Singh', 'amit@abc.com')

Insert into tbl_Students (Firstname, lastname, Email)
 Values('Manish', 'Kumar', 'manish@abc.comm')

Insert into tbl_Students (Firstname, lastname, Email)
 Values('Abhishek', 'Singh', 'abhishek@abc.com')

Now, while writing a Stored Procedure, the first step will be to write the Create Procedure statement as the first statement,

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Create Procedure Procedure-name 
(
Input parameters ,
Output Parameters (If required)
)
As
Begin
     Sql statement used in the stored procedure
End

Now, suppose we need to create a Stored Procedure which will returns a student name whose studentid is given as the input parameter to the stored procedure. Then the Stored Procedure will be:-

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/*  Getstudentname is the name of the stored procedure*/

Create  PROCEDURE Getstudentname(

@studentid INT                   --Input parameter ,  Studentid of the student 

)
AS
BEGIN
SELECT Firstname+' '+Lastname FROM tbl_Students WHERE studentid=@studentid 
END

We can also collect the student name in the output parameter of the Stored Procedure.For Example:

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 /* 
GetstudentnameInOutputVariable is the name of the stored procedure which
uses output variable @Studentname to collect the student name returns by the
stored procedure
*/

Create  PROCEDURE GetstudentnameInOutputVariable
(

@studentid INT,                       --Input parameter ,  Studentid of the student
@studentname VARCHAR(200)  OUT        -- Out parameter declared with the help of OUT keyword
)
AS
BEGIN
SELECT @studentname= Firstname+' '+Lastname FROM tbl_Students WHERE studentid=@studentid
END

Note:-/* */ is used to write comments in one or multiple lines

-- is used to write comment in a single line

How to Alter a Stored Procedure in a Sql server:-

In SQL Server, a stored procedure can be modified with the help of the Alter keyword.Now if, we want to get student email address through the same procedure GetstudentnameInOutputVariable . So we need to modified it by adding one more output parameter " @StudentEmail " which is shown below:

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/* 
Stored Procedure GetstudentnameInOutputVariable is modified to collect the
email address of the student with the help of the Alert Keyword
*/

 

Alter  PROCEDURE GetstudentnameInOutputVariable
(

@studentid INT,                   --Input parameter ,  Studentid of the student
@studentname VARCHAR (200) OUT,    -- Output parameter to collect the student name
@StudentEmail VARCHAR (200)OUT     -- Output Parameter to collect the student email
)
AS
BEGIN
SELECT @studentname= Firstname+' '+Lastname, 
    @StudentEmail=email FROM tbl_Students WHERE studentid=@studentid
END

Note: It is not necessary that a stored procedure will have to written. It can be the case when a storedprocedure doesn't written any thing. For Example, a stored procedure can be used to Insert, delete or update a sql statement. For Example the below stored procedure is used tp insert value into the table tbl_students.

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/*
This Stored procedure is used to Insert value into the table tbl_students. 
*/

Create Procedure InsertStudentrecord
(
 @StudentFirstName Varchar(200),
 @StudentLastName  Varchar(200),
 @StudentEmail     Varchar(50)
) 
As
 Begin
   Insert into tbl_Students (Firstname, lastname, Email)
   Values(@StudentFirstName, @StudentLastName,@StudentEmail)
 End

Execution of the Stored Procedure in SQL Server

Execution of the Stored Procedure which don't have a Output Parameter

A stored procedure is used in the Sql server with the help of the "Execute" or "Exec" Keyword. For Example, If we want to execute the stored procedure "Getstudentname", then we will use the following statement.

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Execute Getstudentname 1
Exec Getstudentname 1

Execution of the Stored Procedure using the Output Parameter

If we want to execute the Stored procedure "GetstudentnameInOutputVariable " , then we first need to declare the variable to collect the output values. For example

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Declare @Studentname as nvarchar(200)   -- Declaring the variable to collect the Studentname
Declare @Studentemail as nvarchar(50)     -- Declaring the variable to collect the Studentemail
Execute GetstudentnameInOutputVariable 1 , @Studentname output, @Studentemail output
select @Studentname,@Studentemail      -- "Select" Statement is used to show the output from Procedure

Summary

In the end we can say that a Stored procedure not only enhancing the possibility of reusing the code and execution plan, but it also increases the performance of the database by reducing the traffic of the network by reducing the amount of information send over the network.