SQL Server - after insert trigger - update another column in the same table

Question

I've got this database trigger:

CREATE TRIGGER setDescToUpper
ON part_numbers
 AFTER INSERT,UPDATE
AS
DECLARE @PnumPkid int, @PDesc nvarchar(128)

SET @PnumPkid = (SELECT pnum_pkid FROM inserted)
SET @PDesc = (SELECT UPPER(part_description) FROM inserted)

UPDATE part_numbers set part_description_upper = @PDesc WHERE pnum_pkid=@PnumPkid

GO

Is this a bad idea? That is to update a column on the same table. I want it to fire for both insert and update.

It works, I'm just afraid of a cyclical situation. The update, inside the trigger, fires the trigger, and again and again. Will that happen?

Please, don't nitpick at the upper case thing. Crazy situation.

Answer 1

It depends on the recursion level for triggers currently set on the DB.

If you do this:

SP_CONFIGURE 'nested_triggers',0
GO
RECONFIGURE
GO

Or this:

ALTER DATABASE db_name
SET RECURSIVE_TRIGGERS OFF

That trigger above won't be called again, and you would be safe (unless you get into some kind of deadlock; that could be possible but maybe I'm wrong).

Still, I do not think this is a good idea. A better option would be using an INSTEAD OF trigger. That way you would avoid executing the first (manual) update over the DB. Only the one defined inside the trigger would be executed.

An INSTEAD OF INSERT trigger would be like this:

CREATE TRIGGER setDescToUpper ON part_numbers
INSTEAD OF INSERT
AS
BEGIN
    INSERT INTO part_numbers (
        colA,
        colB,
        part_description
    ) SELECT
        colA,
        colB,
        UPPER(part_description)
    ) FROM
        INSERTED
END
GO

This would automagically "replace" the original INSERT statement by this one, with an explicit UPPER call applied to the part_description field.

An INSTEAD OF UPDATE trigger would be similar (and I don't advise you to create a single trigger, keep them separated).

Also, this addresses @Martin comment: it works for multirow inserts/updates (your example does not).