I'm not sure if this a performance question or a sql question really, but since my primarily peeve here is performance, here goes:
I'm trying to write a query which takes the output of a join and shows me only what the items that are in the main join but not in the subselect of just one of the tables in the join, using EXCEPT.
This is a little complicated, so please bear with me.
I have two tables: an event table that logs random events as they come in, and a tracking table that keeps a state of events it cares about. In this particular case I'm trying to obtain a list of tracking pkeys for related event data that do not correspond to a certain (other) set of event data.
Ideally, here is what I want:
SELECT tracking.pk,events.data1,events.data2 FROM tracking,events WHERE tracking.event_fk = event.pk EXCEPT (SELECT events.data1,events.data2 FROM events WHERE event.type = 10)
The problem I have of course is that I get an error regarding trying to use different columns for the two queries in EXCEPT. I'm sure someone will point this out, but the following suggestion will not work:
SELECT tracking.pk,events.data1,events.data2 FROM tracking,events WHERE tracking.event_fk = event.pk EXCEPT (SELECT tracking.pk,events.data1,events.data2 FROM tracking,events WHERE tracking.event_fk = event.pk AND event.type = 10)
That won't work for two reasons... first, there are no matching entries in the tracking table pointing to events where event.type = 10, meaning this query would always return an empty set. And even if there were, I don't want to do the join twice if its not necessary, as the events table is liable to be very large.
The official solution to this I believe would be to just use CORRESPONDING BY, but that's not supported by PG (why exactly, oh why!)
Suggestions, anyone? Thanks in advance,
Lucas.
