What is the problem? Do you have NULLs in the the MainZip when it's not present/used?
FROM Doctors D
INNER JOIN ZipCodes Z ON
(D.MainZip IS NOT NULL AND D.MainZip = Z.ZipCode) OR
(D.MainZip IS NULL AND D.AlternateZip = Z.ZipCode)
SQL DBA,SQL Server MVP(07, 08, 09)[size=2]Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial: If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.[/size]