Ken, If I understand the logic correctly, you need a list of institutions with dates after July 1, but only institutions that have records existing before July 1. Subqueries could work, but a view might be easier to work with, especially if you plan to a lot of queries with similar logic.
First, create a view with institutions whose dates are prior to July 1: CREATE VIEW my_list AS SELECT DISTINCT institution FROM renewals WHERE snap_date < '2011-07-01' Then, match the institutions in your renewals table with the ones in the view: SELECT institution FROM renewals a, my_list b WHERE a.institution = b.institution AND snap_date > '2011-07-01' That's the way, at least, I might tackle it... there are probably a dozen others however. ...adam On Sep 28, 2011, at 12:41 PM, Ken Irwin wrote: > 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. > > Thanks > Ken
