RE: SQL update make fields match

Charles (or anyone),

> update TableOne
>   set TableOne.field1 = (select TableTwo.fieldX
>                          from TableTwo
>                          where TableTwo.CommonID = TableOne.CommonID
>                          )

Question on this.  My statement (working) looks like this:

UPDATE IMIMSG
  SET IMIMSG.IMACRC = (SELECT DMITMMST.ACREC
                       FROM DMITMMST
                       WHERE DMITMMST.ITNBR = IMIMSG.IMITEM
                       );

Every row in IMIMSG is updated whether it was changed or not.  Is there a
way to only update it if it needs changing?  There would have to be some
kind of conditional test for this.  In RPG I would check that kind of thing
before updating because it is much more efficient to only update the records
that need it.  Is that true in SQL or, because it is set-at-a-time, is it
more efficient to just do them all?

Thanks for all your help in this.