Groeperen op datum, met 0 wanneer count() geen regels oplevert

Aangezien je de datums niet in de tabel hebt staan, heb je een manier nodig om ze te genereren. U kunt de generate_series gebruiken functie:

SELECT * FROM generate_series('2012-01-01'::timestamp, '2012-01-07 23:00', '1 hour') AS ts;

Dit levert de volgende resultaten op:

         ts          
---------------------
 2012-01-01 00:00:00
 2012-01-01 01:00:00
 2012-01-01 02:00:00
 2012-01-01 03:00:00
...
 2012-01-07 21:00:00
 2012-01-07 22:00:00
 2012-01-07 23:00:00
(168 rows)

De resterende taak is om de twee selects samen te voegen met behulp van een outer join zoals deze:

select extract ( day from ts ) as day, extract ( hour from ts ) as hour,coalesce(count,0) as count from 
(
    SELECT  extract ( day from date ) as day , extract ( hour from date ) as hr ,count(*)
    FROM    sr
    where date>'2012-01-01' and date <'2012-01-07'
    GROUP BY   extract ( day from date ) , extract ( hour from date )
) AS cnt 
 right outer join ( SELECT * FROM generate_series ( '2012-01-01'::timestamp, '2012-01-07 23:00', '1 hour') AS ts ) as dtetable on extract ( hour from ts ) = cnt.hr and extract ( day from ts ) = cnt.day 
 order by day,hour asc;