How Can I Prevent Recursive Triggers in SQL Server?

Preventing Recursive Triggers in SQL Server

In SQL Server 2008, a trigger on the tblMedia table, [dbo].[tblMediaAfterInsertOrUpdate], is inadvertently causing recursive behavior. This behavior occurs when the trigger is both triggered and subsequently triggers itself.

To understand the problem, let's examine the trigger:

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

After Insert or Update: The trigger is executed before inserts or updates are committed.

Potential Recursion: The update statement inside the trigger attempts to modify the tblMedia table. This modification could potentially trigger the same trigger again, leading to an infinite loop.

Preventing Recursion: To prevent this recursive behavior, we can use the TRIGGER_NESTLEVEL() function to check the nesting level of the trigger. If the nesting level is greater than 1, the trigger is being invoked by another trigger and should be skipped.

ALTER TRIGGER [dbo].[tblMediaAfterInsertOrUpdate]
ON [dbo].[tblMedia]
BEFORE INSERT, UPDATE
AS
BEGIN
    IF TRIGGER_NESTLEVEL() <= 1 /* Prevent recursion */
    BEGIN
        ...
    END
END

Explanation:

• The TRIGGER_NESTLEVEL() function returns the number of nested trigger levels. If the current trigger is the outermost trigger (the first to be executed), the nesting level will be 1.
• If the nesting level is greater than 1, the trigger is being invoked by another trigger and should be skipped to avoid recursion.
• If the nesting level is 1, the trigger is not being invoked recursively and can proceed with its normal execution.

By incorporating this check into the trigger, we prevent it from recursing indefinitely and ensure that the necessary data modifications are performed only once.

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