Auditing only updated column with Trigger

Read Prev Project: Auditing Update and Delete Operation with Trigger

When you want to Audit only the column which gets updated, you require something called update () function to find the column which gets updated.

I am writing this article because today I earshot something like this from a conversation which was going between an SQL Expert Manish Sharma and a Project team of iGATE. Their discussion influences me to write up this article. Thanks guys n sorry for listening your discussion;-P, anyways your discussion made an increment to my article list. 

Here I have a table emp with following data.

select * from emp

emp_id      ename      dept_id

----------- ---------- -----------

1           neha       1

2           richa      1

3           sidhika    2

4           sapna      2

5           sneha      3

6           Kavita     3

7           Binny      1

(7 row(s) affected)

Now I am creating a Audit table which will hold the old values of update or delete operation.

Create table Tab_MyAudit1

(    

      A_Action char(1) not null,

      A_AuditDate datetime not null,

      A_Table     varchar(15) not null,  

      A_Values xml

)

Command(s) completed successfully.

My Objective: To write a trigger which will insert only the old column values which gets updated with update statement, no need to record other column values which are not the part of update statement. And in case of delete record all column values.

Now here we are creating a trigger which is using Update() function to check which column gets updated, accordingly it insert the values to Audit table.  The if (SELECT COUNT(*) FROM inserted) > 0 will tell whether the DML operation is Update or Delete. In case of Update INSERTED table will have data so this condition will set to true whereas, in case of delete, only Deleted table will hold data, so this condition will set to false and the pointer will go to else part.

create trigger TGR_EMP_UpdateDeleteLog1

ON emp

for UPDATE,DELETE

AS

begin

      Declare @AuditAction varchar(1)

      Declare @OldMessage varchar(200)

     

      --Condtion for U or D

      if (SELECT COUNT(*) FROM inserted) > 0

      begin

            set @AuditAction = 'U'

            set @OldMessage =

(select  (case

            when update(ename)then deleted.ename end) as OldName,

            (case

when update(dept_id)then deleted.dept_id end) as OldDeptId

            from deleted

            inner join emp

            on deleted.emp_id=emp.emp_id

            FOR XML AUTO)

      end

      else

      begin

            set @AuditAction = 'D'

            set @OldMessage =

            (SELECT * from deleted FOR XML AUTO)

      end

      insert into Tab_MyAudit1 values

      (@AuditAction,getdate(),'EMP',@OldMessage)

End

Command(s) completed successfully.

Now our Trigger is ready, let’s test it with DML statement, before that lets query Audit table and emp table.

select * from Tab_MyAudit

(0 row(s) affected)

So there is no row in Audit table Tab_MyAudit for now. Lets query Emp table too

select * from emp

emp_id      ename      dept_id

----------- ---------- -----------

1           neha       1

2           richa      1

3           sidhika    2

4           Pragyan    2

5           sneha      3

6           Kavita     3

(6 row(s) affected)

Update Operation

Now since everything is in place, let’s fire an Update DML where we are updating employee name to Sneha where the employee id is 4.

update emp

set ename = 'Sneha' where emp_id = 4

let’s check Audit table, the Audit table should record only old Name which gets changed through Update statement. i.e “Pragyan” should get inserted in Audit table.

select * from Tab_MyAudit1

A_Action A_AuditDate             A_Table A_Values

-------- ----------------------- ------- ---------------------------

U        2013-12-30 22:11:26.123 EMP     <deleted OldName="Pragyan"/>

(1 row(s) affected)

So to our expectation we found correct entry in Audit table. Let’s query Emp table too to check the update.

select * from emp

emp_id      ename      dept_id

----------- ---------- -----------

1           neha       1

2           richa      1

3           sidhika    2

4           Sneha      2

5           sneha      3

6           Kavita     3

(6 row(s) affected)

Now we have two employee with employee name ‘Sneha’ but different dept_id i.e. 2 and 3 , lets update the dept Id to 1 where employee name is ‘Sneha’

update emp

set dept_id = 1 where ename = 'Sneha'

Now let’s query  the Audit table, our expectation is, the department which gets updated to 1 are 2 and 3, so 2 and 3 should gets inserted in a single row in audit table.

select * from Tab_MyAudit1

A_Action A_AuditDate A_Table A_Values

-------- ----------- ------- ----------------------------------------------

U        2013-12-30  EMP   <deleted OldName="Pragyan" />

U        2013-12-30  EMP   <deleted OldDeptId="2"/><deleted OldDeptId="3" />

(2 row(s) affected)

So to our expectation it records both the department id as we can see the second record. Also let’s query emp table to check the update.

select * from emp

emp_id      ename      dept_id

----------- ---------- -----------

1           neha       1

2           richa      1

3           sidhika    2

4           Sneha      1

5           sneha      1

6           Kavita     3

(6 row(s) affected)

Delete Operation

Now here we are trying to delete employee with employee id 5 and will check whether the whole record values gets stored in Audit Table.

delete emp where emp_id = 5

Let’s query Audit table

select * from Tab_MyAudit1

A_Action A_AuditDate A_Table A_Values

-------- ---------- -------- ---------------------------------------------

U        2013-12-30 EMP    <deleted OldName="Pragyan" />

U        2013-12-30 EMP    <deleted OldDeptId="2"/><deleted OldDeptId="3"/>

D        2013-12-30 EMP    <deleted emp_id="5" ename="sneha" dept_id="1"/>

(3 row(s) affected)

So to our expectation as we can see the third record, it records all column value in Audit table. Lets query our Employee table to see the update.

  

select * from emp

emp_id      ename      dept_id

----------- ---------- -----------

1           neha       1

2           richa      1

3           sidhika    2

4           Sneha      1

6           Kavita     3

(5 row(s) affected)

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