Grouping by date, with 0 when count() yields no lines

Question

I\'m using Postgresql 9 and I\'m fighting with counting and grouping when no lines are counted.

Let\'s assume the following schema :

create table vie         


        

Answer 1

Given that you don't have the dates in the table, you need a way to generate them. You can use the generate_series function:

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

This will produce results like this:

         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)

The remaining task is to join the two selects using an outer join like this :

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;

Answer 2

This query will give you the output what your are looking for,

select to_char(date_event, 'YYYY-MM-DD HH24:00') as time, count (to_char(date_event, 'HH24:00')) as count from views where date(date_event) > '2012-01-01' and date(date_event) > '2012-01-07' group by time order by time;