Access Variable Inside a Trigger

Have you ever needed to pass a variable into a trigger? So far, I have not encountered the need to pass a variable to a trigger. I figure, sooner or later, I am going to encounter a similar requirement. And while I can still remember this simple workaround, might as well post it.

The idea is to take advantage of the CONTEXT_INFO statement. CONTEXT_INFO is a statement that returns the context_info value that was set for the current session or batch.

Here’s the remarks excerpt from the BOL:

Multiple active result sets (MARS) enables applications to run multiple batches, or requests, at the same time on the same connection. When one of the batches on a MARS connection runs SET CONTEXT_INFO, the new context value is returned by the CONTEXT_INFO function when it is run in the same batch as the SET statement. The new value is not returned by the CONTEXT_INFO function run in one or more of the other batches on the connection, unless they started after the batch that ran the SET statement completed. 

To illustrate, consider this table:

create table students
(
 StudentId tinyint identity(1,1),
 StudentName varchar(25)
)
insert into students (studentname)
select 'John'
union all
select 'Paul'
union all 
select 'George'

select * from students

This is how the table looks like:

StudentId StudentName
--------- -------------------------
1         John
2         Paul
3         George

Now let's create the trigger..

create trigger trgDelete
on students for delete
as 
begin
 insert into students (studentname)
 values (cast(CONTEXT_INFO() as varchar(10)))
end

Here's the sample code to illustrate...

declare @namevar varchar(10)
DECLARE @BinVar varbinary(128)

set @namevar = 'Ringo'
SET @BinVar = CAST(@namevar AS varbinary(128) )
SET CONTEXT_INFO @BinVar

delete from students 
where studentname = 'Paul'

select * from students

Here is the result after the delete statement...

StudentId StudentName
--------- -------------------------
1         John
4         Ringo     
3         George

This is just a sample that shows how a string can be access inside a trigger. Other data type may also be stored, just don't forget that the value needs to be converted into a varbinary, then just convert it back as needed.
As always, comments are welcome...


~~ CK