Count full months between two dates - sql

Question

I've been working on this for a few hours with no luck and have hit a wall. My data looks like this:
Date1 Date2
2012-05-06 2012-05-05
2012-03-20 2012-01-05
What I'm trying to do is add 1 to the count for every month between two dates. So my output would ideally look like this:
Year Month Sum
2012 2 1
In other words, it should check for "empty" months between two dates and add 1 to them.
This is the code I've worked out so far. It will basically count the number of months between the two dates and group them into months and years.
SELECT
EXTRACT(YEAR FROM Date2::date) as "Year",
EXTRACT(MONTH FROM Date2::date) as "Month",
SUM(DATE_PART('year', Date1::date) - DATE_PART('year', Date2::date)) * 12 +
(DATE_PART('month', Date1::date) - DATE_PART('month', Date2::date))
FROM
test
GROUP BY
"Year",
"Month",
ORDER BY
"Year" DESC,
"Month" DESC;
This is where I'm stuck - I don't know how to actually add 1 for each of the "empty" months.

Answer 1

Test setup
With some sample rows (should be provided in the question):
CREATE TABLE test (
test_id serial PRIMARY KEY
, date1 date NOT NULL
, date2 date NOT NULL
);
INSERT INTO test(date1, date2)
VALUES
('2012-03-20', '2012-01-05') -- 2012-02 lies in between
, ('2012-01-20', '2012-03-05') -- 2012-02 (reversed)
, ('2012-05-06', '2012-05-05') -- nothing
, ('2012-05-01', '2012-06-30') -- still nothing
, ('2012-08-20', '2012-11-05') -- 2012-09 - 2012-10
, ('2012-11-20', '2013-03-05') -- 2012-12 - 2013-02
;
Postgres 9.3 or newer
Use a LATERAL join:
SELECT to_char(mon, 'YYYY') AS year
, to_char(mon, 'MM') AS month
, count(*) AS ct
FROM (
SELECT date_trunc('mon', least(date1, date2)::timestamp) + interval '1 mon' AS d1
, date_trunc('mon', greatest(date1, date2)::timestamp) - interval '1 mon' AS d2
FROM test
) sub1
, generate_series(d1, d2, interval '1 month') mon -- implicit CROSS JOIN LATERAL
WHERE d2 >= d1 -- exclude ranges without gap right away
GROUP BY mon
ORDER BY mon;
What is the difference between LATERAL and a subquery in PostgreSQL?
Postgres 9.2 or older
No LATERAL, yet. Use a subquery instead:
SELECT to_char(mon, 'YYYY') AS year
, to_char(mon, 'MM') AS month
, count(*) AS ct
FROM (
SELECT generate_series(d1, d2, interval '1 month') AS mon
FROM (
SELECT date_trunc('mon', least(date1, date2)::timestamp) + interval '1 mon' AS d1
, date_trunc('mon', greatest(date1, date2)::timestamp) - interval '1 mon' AS d2
FROM test
) sub1
WHERE d2 >= d1 -- exclude ranges without gap right away
) sub2
GROUP BY mon
ORDER BY mon;
Result
year | month | ct
------+-------+----
2012 | 2 | 2
2012 | 9 | 1
2012 | 10 | 1
2012 | 12 | 1
2013 | 1 | 1
2013 | 2 | 1
db<>fiddle here
SQL Fiddle.
Explanation
You are looking for complete calendar months between the two dates.
These queries work with any dates or timestamps in ascending or descending order and should perform well.
The WHERE clause is optional, since generate_series() returns no row if start > end. But it should be a bit faster to exclude empty ranges a priori.
The cast to timestamp makes it a bit cleaner and faster. Rationale:
Generating time series between two dates in PostgreSQL

Answer 2

AFAIK you can simply substract/add dates in postgresql
'2001-06-27 14:43:21'::DATETIME - '2001-06-27 14:33:21'::DATETIME = '00:10:00'::INTERVAL
So in your case that request part should look like
DATE_PART('month', Date1::datetime - Date2::datetime) as "MonthInterval"

Answer 3

age(timestamp1, timestamp2) => returns interval
the we try to extract year and month out of the interval and add them accordingly.
select extract(year from age(timestamp1, timestamp2))*12 + extract(month from
age(timestamp1, timestamp2))