postgresql daysdiff between two dates grouped by month - postgresql

Question

I have a table with the date columns (start_date, end_date) and I want to calculate the difference between these dates and grouped by the month.
I am able to get the datediff in days, but I do not know how to group this in month, any suggestions?
Table:
id Start_date End_date days
1234 2014-06-03 2014-07-05 32
12345 2014-02-02 2014-05-10 97
Expected results:
month diff_days
2 26
3 30
4 31
5 10
6 27
7 5

Answer 1

I think your expected output numbers are off a little. You might want to double-check.
I use a calendar table myself, but this query uses a CTE and date arithmetic. Avoiding the hard-coded date '2014-01-01' and the interval for 365 days is straightforward, but it makes the query harder to read, so I just used those values directly.
with your_data as (
select date '2014-06-03' as start_date, date '2014-07-05' as end_date union all
select '2014-02-02', '2014-05-10'
), calendar as (
select date '2014-01-01' + (n || ' days')::interval calendar_date
from generate_series(0, 365) n
)
select extract (month from calendar_date) calendar_month, count(*) from calendar
inner join your_data on calendar.calendar_date between start_date and end_date
group by calendar_month
order by calendar_month;
calendar_month count
--
2 27
3 31
4 30
5 10
6 28
7 5
As a rule of thumb, you should never group by the month alone--doing that risks grouping data from different years. This is a safer version that includes the year, and which also restricts output to a single calendar year.
with your_data as (
select date '2014-06-03' as start_date, date '2014-07-05' as end_date union all
select '2014-02-02', '2014-05-10'
), calendar as (
select date '2014-01-01' + (n || ' days')::interval calendar_date
from generate_series(0, 700) n
)
select extract (year from calendar_date) calendar_year, extract (month from calendar_date) calendar_month, count(*) from calendar
inner join your_data on calendar.calendar_date between start_date and end_date
where calendar_date between '2014-01-01' and '2014-12-31'
group by calendar_year, calendar_month
order by calendar_year, calendar_month;

Answer 2

SQL Fiddle
with min_max as (
select min(start_date) as start_date, max(end_date) as end_date
from t
), g as (
select daterange(d::date, (d + interval '1 month')::date, '[)') as r
from generate_series(
(select date_trunc('month', start_date) from min_max),
(select end_date from min_max),
'1 month'
) g(d)
)
select *
from (
select
to_char(lower(r), 'YYYY Mon') as "Month",
sum(upper(r) - lower(r)) as days
from (
select t.r * g.r as r
from
(
select daterange(start_date, end_date, '[]') as r
from t
) t
inner join
g on t.r && g.r
) s
group by 1
) s
order by to_timestamp("Month", 'YYYY Mon')
;
Month | days
----------+------
2014 Feb | 27
2014 Mar | 31
2014 Apr | 30
2014 May | 10
2014 Jun | 28
2014 Jul | 5
Range data types
Range functions and operators