Postgresql group month wise with missing values

Question

first an example of my table:

id_object;time;value;status
1;2014-05-22 09:30:00;1234;1
1;2014-05-22 09:31:00;2341;2
1;2014-05-22 09:32:00;1234;1
...
1;2014-0         


        

Answer 1

you can use generate_series() function like this:

select
    g.month,
    count(m)
from generate_series(1, 12) as g(month)
    left outer join my_table as m on
        m.id_object = 1 and
        m.status = 1 and
        extract(year from m.time) = 2014 and
        extract(month from m.time) = g.month
group by g.month
order by g.month

sql fiddle demo

Answer 2

Rather than comparing with an extracted value, you'll want to use a range-table instead. Something that looks like this:

month  startOfMonth  nextMonth
1      '2014-01-01'  '2014-02-01'
2      '2014-02-01'  '2014-03-01'
......
12     '2014-12-01'  '2015-01-01'

As in @Roman's answer, we'll start with generate_series(), this time using it to generate the range table:

WITH Month_Range AS (SELECT EXTRACT(MONTH FROM month) AS month, 
                            month AS startOfMonth,
                            month + INTERVAL '1 MONTH' AS nextMonth
                     FROM generate_series(CAST('2014-01-01' AS DATE),
                                          CAST('2014-12-01' AS DATE),
                                          INTERVAL '1 month') AS mr(month))
SELECT Month_Range.month, COUNT(My_Table) 
FROM Month_Range
LEFT JOIN My_Table
       ON My_Table.time >= Month_Range.startOfMonth
          AND My_Table.time < Month_Range.nextMonth
          AND my_table.id_object = 1
          AND my_table.status = 1
GROUP BY Month_Range.month
ORDER BY Month_Range.month

(As a side note, I'm now annoyed at how PostgreSQL handles intervals)

SQL Fiddle Demo

The use of the range will allow any index including My_Table.time to be used (although not if an index was built over an EXTRACTed column.

EDIT:

Modified query to take advantage of the fact that generate_series(...) will also handle date/time series.

Answer 3

generate_series can generate timestamp series

select
    g.month,
    count(t)
from
    generate_series(
        (select date_trunc('year', min(t.time)) from t),
        (select date_trunc('year', max(t.time)) + interval '11 months' from t),
        interval '1 month'
    ) as g(month)
    left outer join
    t on
        t.id_object = 1 and
        t.status = 1 and
        date_trunc('month', t.time) = g.month
where date_trunc('year', g.month) = '2014-01-01'::date
group by g.month
order by g.month