>>But what is the logic... Would something like this work?
>>
>>

>>CREATE TRIGGER CheckPK ON [Product]
>>FOR UPDATE
>>AS
>>DECLARE @countFk int
>>SELECT @countFk = SELECT count(fk_product) from Traveler where fk_product = insert.pk)
>>
>>IF @countFk > 0
>>	AND (fk_engine != insert.fk_engine
>>	OR fk_stage != insert.fk_stage
>>	OR fk_part != insert.fk_part)
>>IF EXISTS ( SELECT * FROM inserted i JOIN DELETED d ON d.pk = i.pk 
				WHERE d.fk_engine <> i.fk_engine 
					OR d.fk_stage <> i.fk_stage 
					OR d.fk_part <> i.fk_part)


>>
>>

>
>Margaret,
>
>I think you should check the previous value (e.g. from deleted) and then may be your logic against inserted table, though I'm not sure what exactly you're checking.
>
>In other words, it should check
>

>SELECT @countFk = SELECT count(fk_product) from Traveler where fk_product = deleted.pk)

>
>and then act accordingly. Also do you want to check if you changed fk_engine, etc. fields? If yes, then join deleted with inserted tables based on the PK.

Combining yours and Sergey's idea it would be:

 if exists (select * from Traveler where fk_product = deleted.PK) and 
  EXISTS ( SELECT * FROM inserted i JOIN DELETED d ON d.pk = i.pk 
				WHERE d.fk_engine <> i.fk_engine 
					OR d.fk_stage <> i.fk_stage 
					OR d.fk_part <> i.fk_part)

  begin
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  end