-- #1 SELECT * FROM jw2_studemo_suniq_ident WHERE ident NOT IN (SELECT ident FROM catcboe_students) -- #2 Correlated subquery SELECT * FROM jw2_studemo_suniq_ident v1 WHERE NOT Exists (SELECT * FROM catcboe_students cs WHERE cs.ident = v1.ident ) -- #3 SELECT v1.* FROM jw2_studemo_suniq_ident v1 LEFT OUTER JOIN catcboe_students cs WHERE v1.ident = cs.ident AND cs.ident IS NULL )You should run some tests to find out which one is the fastest. My guess would be that it's #2.