Re: Aggregates

John D. Burger wrote:
>
> 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;

Ah, but this just includes the time of the last message, not its data.

--
   Richard Huxton
   Archonet Ltd