>>>OK, thanks again. I'll try the concatenation.
>>>
>>Why not leave it as a JOIN?
>
>Yeah, perhaps I'll do that. I guess it should work both for IN and for NOT IN (checking for null values).
>
>Mainly, I was wondering whether the subquery could be extended to this case, but it seems it isn't possible with a subquery as such. (EXISTS is already another kind of clause...)

You may want to review Re: Bug in SQL Union Thread #1199863