How do I prevent a database trigger from recursing

Question

I've got the following trigger on a table for a SQL Server 2008 database. It's recursing, so I need to stop it.

After I insert or update a record, I'm trying to simply update a single field on that table.

Here's the trigger :

ALTER TRIGGER [dbo].[tblMediaAfterInsertOrUpdate] 
   ON  [dbo].[tblMedia]
   BEFORE INSERT, UPDATE
AS 
BEGIN
    SET NOCOUNT ON

    DECLARE @IdMedia INTEGER,
        @NewSubject NVARCHAR(200)   

    SELECT @IdMedia = IdMedia, @NewSubject = Title
    FROM INSERTED

    -- Now update the unique subject field.
    -- NOTE: dbo.CreateUniqueSubject is my own function. 
    --       It just does some string manipulation.
    UPDATE tblMedia
    SET UniqueTitle = dbo.CreateUniqueSubject(@NewSubject) + 
                      CAST((IdMedia) AS VARCHAR(10))
    WHERE tblMedia.IdMedia = @IdMedia
END

Can anyone tell me how I can prevent the trigger's insert from kicking off another trigger again?

Answer 1

I see three possibilities:

1. Disable trigger recursion:

   This will prevent a trigger fired to call another trigger or calling itself again. To do this, execute this command:

   ALTER DATABASE MyDataBase SET RECURSIVE_TRIGGERS OFF
   GO
   

2. Use a trigger INSTEAD OF UPDATE, INSERT

   Using a INSTEAD OF trigger you can control any column being updated/inserted, and even replacing before calling the command.

3. Control the trigger by preventing using IF UPDATE

   Testing the column will tell you with a reasonable accuracy if you trigger is calling itself. To do this use the IF UPDATE() clause like:

   ALTER TRIGGER [dbo].[tblMediaAfterInsertOrUpdate]
      ON  [dbo].[tblMedia]
      FOR INSERT, UPDATE
   AS
   BEGIN
       SET NOCOUNT ON
       DECLARE @IdMedia INTEGER,
           @NewSubject NVARCHAR(200)   
   
       IF UPDATE(UniqueTitle)
         RETURN;
   
       -- What is the new subject being inserted?
       SELECT @IdMedia = IdMedia, @NewSubject = Title
       FROM INSERTED
   
       -- Now update the unique subject field.
       -- NOTE: dbo.CreateUniqueSubject is my own function. 
       --       It just does some string manipulation.
       UPDATE tblMedia
       SET UniqueTitle = dbo.CreateUniqueSubject(@NewSubject) + 
                         CAST((IdMedia) AS VARCHAR(10))
       WHERE tblMedia.IdMedia = @IdMedia
   END
   

Answer 2

TRIGGER_NESTLEVEL can be used to prevent recursion of a specific trigger, but it is important to pass the object id of the trigger into the function. Otherwise you will also prevent the trigger from firing when an insert or update is made by another trigger:

   IF TRIGGER_NESTLEVEL(OBJECT_ID('dbo.mytrigger')) > 1
         BEGIN
             PRINT 'mytrigger exiting because TRIGGER_NESTLEVEL > 1 ';
             RETURN;
     END;

From MSDN:

When no parameters are specified, TRIGGER_NESTLEVEL returns the total number of triggers on the call stack. This includes itself.

Reference: Avoiding recursive triggers

Answer 3

For completeness sake, I will add a few things. If you have a particular after trigger that you only want to run once, you can set it up to run last using sp_settriggerorder.

I would also consider if it might not be best to combine the triggers that are doing the recursion into one trigger.

Answer 4

You can have a separate NULLABLE column indicating whether the UniqueTitle was set.

Set it to true value in a trigger, and have the trigger do nothing if it's value is true in "INSERTED"

Answer 5

ALTER DATABASE <dbname> SET RECURSIVE_TRIGGERS OFF

RECURSIVE_TRIGGERS { ON | OFF }

ON Recursive firing of AFTER triggers is allowed.

OFF Only direct recursive firing of AFTER triggers is not allowed. To also disable indirect recursion of AFTER triggers, set the nested triggers server option to 0 by using sp_configure.

Only direct recursion is prevented when RECURSIVE_TRIGGERS is set to OFF. To disable indirect recursion, you must also set the nested triggers server option to 0.

The status of this option can be determined by examining the is_recursive_triggers_on column in the sys.databases catalog view or the IsRecursiveTriggersEnabled property of the DATABASEPROPERTYEX function.

Answer 6

I think i got it :)

When the title is getting 'updated' (read: inserted or updated), then update the unique subject. When the trigger gets ran a second time, the uniquesubject field is getting updated, so it stop and leaves the trigger.

Also, i've made it handle MULTIPLE rows that get changed -> I always forget about this with triggers.

ALTER TRIGGER [dbo].[tblMediaAfterInsert] 
   ON  [dbo].[tblMedia]
   FOR INSERT, UPDATE
AS 
BEGIN
    SET NOCOUNT ON

    -- If the Title is getting inserted OR updated then update the unique subject.
    IF UPDATE(Title) BEGIN
        -- Now update all the unique subject fields that have been inserted or updated.
        UPDATE tblMedia 
        SET UniqueTitle = dbo.CreateUniqueSubject(b.Title) + 
                          CAST((b.IdMedia) AS VARCHAR(10))
        FROM tblMedia a
            INNER JOIN INSERTED b on a.IdMedia = b.IdMedia
    END
END