Help: SQL .*Not IN* alternatives

I have a user with two flat files who is trying to find data in one table that is not in the other. The tables look like the following (most columns have been omitted): IDMS (6,000 rows) NCS (260,000 rows) table table -------------- ----------- 333-33-3333 333-33-3333 Anderson, Paul Anderson, Paul 444-44-4444 555-55-5555 Smith, Ed Plicer, Felix 555-55-5555 777-77-7777 Gordon, Samual Smith, John Q 666-66-6666 888-88-8888 Powell, George Frederick, Emmett 888-88-8888 Frederick, Emmett He wants a list of all ssn's in table IDMS and NOT IN table NCS. Result set should be: 444-44-4444 (plus other columns) 666-66-6666 ( " ) He used the following SQL: SELECT DISTINCT "idms"."ssn", "idms"."lastname", "idms"."firstname", "idms"."mi", "idms"."ssaciid", "idms"."college", "idms"."initials" FROM "idms", "ncs" WHERE ( "idms"."ssn" NOT IN (SELECT DISTINCT "ncs"."ssn" from "ncs")) ORDER BY "idms"."college" ASC but since these files are large, it was still chugging along 4 hours after the query was started. One reason why it was taking so long was mainly due to the query running in a local version of WATCOM's SQL database. (Note: these tables will eventually find their way into an Informix database just as soon as I can get all the pieces/parts to talk to each other.) Since this appears to be a common problem that we will run into when converting data, I would be interested in hearing better solutions. I seem to remember a previous discussion about this here, but I've had no luck finding it. TIA, Don Boothby (donald_boothby@ima.isd.state.in.us)