PGexercises.com | Count days in each month

-- https://pgexercises.com/questions/date/daysinmonth.html
-- date_part does not work for the second column of the second clause, but date_trunc does

with gs as (select generate_series(
      '2012-01-01'::timestamp,
      '2012-12-31'::timestamp,
      '1 month'::interval) as month)

select date_part('month', month) as m,
        (date_trunc('month', month)+'1 month'::INTERVAL)-date_trunc('month', month) as len
from gs