[dba-SQLServer] Processing diverse where clauses

I am processing a query where a set of N fields have any value, AND one
specific field has an IN() clause, i.e. a ton of codes possible.

Is it more efficient to build up a pair of queries, SELECT PKID from tblX
WHERE FldA in('A','B'...), and another query where the "OR" fields are
gathered?  Or just use one big query?  I hate to even press the button...  I
am having to test with a TOP 100 kind of thing as it is.

John W. Colby
Colby Consulting
www.ColbyConsulting.com