Hi all,

I've not done much with MySQL subqueries, and I'm trying right now with what I find to be surprising results. I wonder if someone can help me understand.

I have a pile of data that with columns for "institution" and "date". Institution gets repeated a lot, with many different dates. I want to select all the institutions that *only* have dates after July 1 and don't appear in the table before that. My solution was to do a first query for all the institutions that DO have dates before July 1
SELECT distinct institution FROM `renewals` WHERE snap_date < '2011-07-01'

And then to do a SELECT query on all the institutions:
SELECT distinct institution from renewals

And then try to do a NOT IN subquery subtracting the smaller query from the larger one:

SELECT distinct institution from renewals
WHERE institution not in
(SELECT distinct institution FROM `renewals` WHERE snap_date < '2011-07-01')

...only it doesn't seem to work. Or rather, the query has been running for several minutes and never comes back with an answer. Each of these two queries takes just a few milliseconds to run on its own.

Can someone tell me (a) am I just formatting the query wrong, (b) do subqueries like this just take forever, and/or (c) is there a better way to do this? (I don't really understand about JOIN queries, but from what I can tell they are only for mixing the results of two different tables so I think they might not apply here.)

Any advice would be most welcome.