Question

I have written a small PostgreSQL query that helps me total amount of jobs executed per hourly intervals in every day within two certain dates -e.g. all the jobs executed between February 2, 2012 and March 3, 2012 hour by hour starting with the hour given in February 2 and ending with the hour given in March 3- I have noticed that this query doesn’t print the rows with 0 count -no job executed within that time interval e.g. at February 21, 2012 between 5 and 6pm-. How can I make this also return results(rows) with 0 count? The code is as below:

SELECT date_trunc('hour', executiontime), count(executiontime)
  FROM mytable
 WHERE executiontime BETWEEN '2011-2-2 0:00:00' AND '2012-3-2 5:00:00' 
 GROUP BY date_trunc('hour', executiontime)
 ORDER BY date_trunc('hour', executiontime) ASC;

Thanks in advance.

Answer 1

        -- CTE to the rescue!!!
WITH cal AS (
        SELECT generate_series('2012-02-02 00:00:00'::timestamp , '2012-03-02 05:00:00'::timestamp , '1 hour'::interval) AS stamp
        )
, qqq AS (
        SELECT date_trunc('hour', executiontime) AS stamp
        , count(*) AS zcount
        FROM mytable
        GROUP BY date_trunc('hour', executiontime)
        )
SELECT cal.stamp
        , COALESCE (qqq.zcount, 0) AS zcount
FROM cal
LEFT JOIN qqq ON cal.stamp = qqq.stamp
ORDER BY stamp ASC
        ;