Informix Help

I remember there was a way starting with version 4.1 where you could do a select using the "in" and "not in" functionallity on multiple fields. For example table a x char(10) y smallint table b x char(10) y smallint You want to find all the rows in table a were there is an "x in table a and no coorresponding "x" in table b the query would be: select rowid from a where x not in (select x from b) Now the question - how do you find all the x and y combinations in table a where they are not in table b. I thought the syntax was something like: select rowid from a where x || y not in (select x || y from b) But I can't seem to make it work or find it in the manual... Anyone else seen this???? Please post. Using Version 5.0 Standard Engine on Sun. Thanks Stuart Litel