maybe is a stupid question.. about CASE

I have a little query in access ..now i must write it in T-Sql

the original...
(MSG.IDLINGUA = IIF( EXISTS
(SELECT * FROM ESMESSAGGI AS MSG WHERE MSG.IDLINGUA = [@.idLingua] AND MSG.IDMESSAGGIO = BITS.IDDESCR),
[@.idLingua], LDEF.IDLINGUA))

I can't be able to assign a select value at my field... IDLINGUA to perform another select..
Of course I have used the "select case when exists"

Anyone could be help me ?Post the code you tried that failed.|||By the reference on your where clause to a table or alias called "Bits", it seems this is part of a larger query.
Please, post your whole query.