INSTEAD OF UPDATE Trigger - is this possible?

Question

I am making some tweaks to a legacy application built on SQL Server 2000, needless to say I only want to do the absolute minimum in the fear that it may just all fall apart.

Answer 1

Using the UPDATE(columnname) test, you can check in a trigger whether a specific column was updated (and then take specific actions), but you can't have a trigger fire only on the update of a specific column. It will fire as soon as the update is performed, regardless of the fact which column was the target of the update.

So, if you think you have to use an INSTEAD OF UPDATE trigger, you'll need to implement two kinds of actions in it:

1) insert into tbDeletedUsers + delete from tbUsers – when IsDeleted is updated (or, more exactly, updated and set to 1);

2) update tbUsers normally – when IsDeleted is not updated (or updated but not set to 1).

Because more than one row can be updated with a single UPDATE instruction, you might also need to take into account that some rows might have IsDeleted set to 1 and others not.

I'm not a big fan of INSTEAD OF triggers, but if I really had to use one for a task like yours, I might omit the UPDATE() test and implement the trigger like this:

CREATE TRIGGER trg_ArchiveUsers
ON tbUsers
INSTEAD OF UPDATE
AS
BEGIN
  UPDATE tbUsers
  SET
    column = INSERTED.column,
    …
  FROM INSERTED
  WHERE INSERTED.key = tbUsers.key
    AND INSERTED.IsDeleted = 0
  ;
  DELETE FROM tbUsers
  FROM INSERTED
  WHERE INSERTED.key = tbUsers.key
    AND INSERTED.IsDeleted = 1
  ;
  INSERT INTO tbDeletedUsers (columns)
  SELECT columns
  FROM INSERTED
  WHERE IsDeleted = 1
  ;
END