Postgresql group month wise with missing values - sql

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;2014-06-01 00:00:00;4321;1
...
Now i need count all rows with status=1 and id_object=1 monthwise for example. this is my query:
SELECT COUNT(*)
FROM my_table
WHERE id_object=1
AND status=1
AND extract(YEAR FROM time)=2014
GROUP BY extract(MONTH FROM time)
The result for this example is:
2
1
2 for may and 1 for june but i need a output with all 12 months, also months with no data. for this example i need this ouput:
0 0 0 0 2 1 0 0 0 0 0 0
Thx for help.

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