Data tweaking code runs fine when executed directly - but never stops when used in trigger

Question

I have written some code to ensure that items on an order are all numbered (the "position number" or "item number" has been introduced only recently and we did not want to go and change all related code - as it is "asthetics only" and has no functional impact.)

So, the idea is to go and check for an records that jave an itemno of NULL or 0 - and then compute one and assign it. When executing this code in a query window, it works fine. When putting it into an AFTER INSERT-trigger, it loops forever.

So what is wrong here?

/****** Objekt:  Trigger [SetzePosNr]    Skriptdatum: 02/28/2010 20:06:29 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE TRIGGER [SetzePosNr]
ON [dbo].[bestellpos]
   AFTER INSERT 
  AS
BEGIN
   DECLARE @idb int
   DECLARE @idp int
   DECLARE @pnr int

   SELECT @idp=id,@idb=id_bestellungen FROM bestellpos WHERE posnr IS NULL OR posnr=0

   WHILE @idp IS NOT NULL
   BEGIN   
      SELECT @pnr = 1+max(posnr) FROM bestellpos WHERE id_bestellungen = @idb
print( 'idp=' + str(@idp) + ', idb=' + str(@idb) + ', posnr=' + str(@pnr))
      UPDATE bestellpos SET posnr=@pnr WHERE id=@idp
      SELECT @idp=id,@idb=id_bestellungen FROM bestellpos WHERE posnr IS NULL OR posnr=0
   END
 END

Answer 1

Aaaargh - just found the problem: the 3d line (from the end, the SELECT-statement) does not assign the variables when there result of the WHERE is NULL. So "set @idp=null" before that statement has fixed the problem! Not sure whhy I assumed a problem between direct execution and triggering of these statements, seems I picked the wrong test-cases :(