Обсуждение: Three fields table: id-data-date_time, how to get max() and date_time same time?

zxo102 ouyang wrote:
> Hi everyone,
>    I have a table "test" which has three fields:
>
> id      data               date_time
> 1        2         2009-10-1 12:12:12
> 1        10       2009-10-1 12:22:10
> 2        3         2009-10-1 12:10:32
> 2        1         2009-10-1 12:30:32
>
> with the sql:
>
> select max(data), id from test where 1=1 group by id
>
> I can get
>
> 10     1
>  3      2
>
> but I want corresponding "date_time" like
>
> 10     1   2009-10-1 12:22:10
>  3      2   2009-10-1 12:10:32
>
> if I use the sql below
>
> select max(data), id, date_time from test where 1=1 group by id, date_time
>
> that won't give the correct results I want.

This seems to produce the right results:

select max(data), id, (select max(date_time) from test where id=t1.id)
from test t1 group by id;

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On Fri, Nov 06, 2009 at 02:09:03PM +1100, Chris wrote:
> select max(data), id, (select max(date_time) from test where id=t1.id)
> from test t1 group by id;

I'd tend to use the DISTINCT ON[1] operator for these sorts of problems:

  SELECT DISTINCT ON (id) *
  FROM test
  ORDER BY id, data DESC;

--
  Sam  http://samason.me.uk/

 [1] http://www.postgresql.org/docs/current/static/sql-select.html#SQL-DISTINCT