On Jun 21, 2007, at 09:22, Richard Huxton wrote:
Naz Gassiep wrote:
Hi,
If I have a table with users and a table with messages, is it
possible to have a query that returns user.* as well as one extra
column
with the number of messages they have posted and the data and time of
the last message? At the moment I am using a subquery to do this,
however it seems suboptimal. Is there a better way?
Not really. You have three separate queries really:
1. User details
2. Total number of messages posted
3. Details on last message posted
Unless you have a messaging-summary table that you keep up-to-date
with triggers you're looking at a three-part query.
Certainly except for the user details it could be a single GROUP BY
with several aggregate functions, something like:
select user.userid, count(*), max(message.datetime)
from user join message using (userid)
group by user.userid;
But if userid is UNIQUE, then so is user.*. You can't do something
like GROUP BY USER.*, but you can group by all the user columns
you're actually interested in selecting:
select userid, user.name, user.address, count(*), max
(message.datetime)
from user join message using (userid)
group by userid, user.name, user.address;
As to whether this is faster or prettier than a subquery, I dunno.
- John D. Burger
MITRE
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