Date: Wed, 26 Oct 2005 10:15:58 -0600
From: Michael Fuhr <mike@fuhr.org>
To: Volkan YAZICI <volkan.yazici@gmail.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Combining two SELECTs by same filters
Message-ID: <20051026161558.GA44965@winnie.fuhr.org>
References: <7104a7370510260816l9262d19j808d23c023e8445e@mail.gmail.com>
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In-Reply-To: <7104a7370510260816l9262d19j808d23c023e8445e@mail.gmail.com>

On Wed, Oct 26, 2005 at 06:16:13PM +0300, Volkan YAZICI wrote:
> => SELECT
> ->     (SELECT count(id) FROM sales
> ->         WHERE id = 2
> ->         AND date_trunc('hour', dt) = '2005-10-25 21:00:00'),
> ->     (SELECT count(id) FROM sales
> ->         WHERE id = 2
> ->         AND date_trunc('hour', dt) = '2005-10-25 22:00:00');
>  ?column? | ?column?
> ----------+----------
>         6 |        2
> (1 row)
> 
> Isn't it possible to combine these two SELECTs as one.

If you can accept multiple rows instead of multiple columns then
one way would be to group by the hour:

SELECT date_trunc('hour', dt) AS hour, count(*)
FROM sales
WHERE id = 2
  AND date_trunc('hour', dt) IN ('2005-10-25 21:00:00', '2005-10-25 22:00:00')
GROUP BY hour
ORDER BY hour;
        hour         | count 
---------------------+-------
 2005-10-25 21:00:00 |     6
 2005-10-25 22:00:00 |     2
(2 rows)

Here's another possibility, but I find it a bit ugly:

SELECT sum(CASE date_trunc('hour', dt)
             WHEN '2005-10-25 21:00:00' THEN 1
             ELSE 0
           END) AS count1,
       sum(CASE date_trunc('hour', dt)
             WHEN '2005-10-25 22:00:00' THEN 1
             ELSE 0
           END) AS count2
FROM sales
WHERE id = 2;
 count1 | count2 
--------+--------
      6 |      2
(1 row)

If you're looking for the fastest method then use EXPLAIN ANALYZE
on each to see what works best on your data set.

-- 
Michael Fuhr