Raggruppamento per data, con 0 quando count() non produce righe

Dato che non hai le date nella tabella, hai bisogno di un modo per generarle. Puoi utilizzare generate_series funzione:

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

Questo produrrà risultati come questo:

         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)

L'attività rimanente è unire le due selezioni utilizzando un join esterno come questo:

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;