It would help if you could provide the execution plans for both statements.

The main benefit for a subquery is to 'flatten' M:1 relations. For example, if I want some information about employees that worked on a given project, and this information is in a bunch of tables, it will probably be better to join all those tables in the main query and just do a WHERE EMP_ID IN ( SELECT DISTINCT EMP_ID FROM TIMECARD WHERE PROJECT_CD = :PROJECT_CD). This is because the timecard table is going to have *tons* of entries for each employee that used that project code. If I join that table through the whole query, then my entire resultset size will be hundreds of times larger than it has to be up until the point of doing some sum or distinct or group by at the outer-most level. If I move the DISTINCT (or sum or whatever) into the inner-most level, then I may save a lot of time building the rest of the resultset.

With an IN, even without the DISTINCT, you will end up with a smaller resultset when you're done. Since you are doing a group by at the end of the first query, I will assume that this is the case. You should probably add the DISTINCT keyword in whichever of the INs is actually returning duplicate IDs (or both). This will make the functionality more obvious and also improve performance, since it will not need to search on the same ID twice for inclusion.

HTH,

- Chris