If its comma separated you will need to use Split function to use IN query
How to Split a string Value in Sql Server 2005
so that it splits a string and converts it to a Table and your query might look something like this, (it is a psuedo-code)
SELECT AssetId,AssetName FROM Table1 WHERE SubCategory='MCC' and AssetId NOT EXISTS (SELECT dbo.SplitString(AssetId, ',') FROM Table2)