How do I convert a monthly report to a quarterly report in PostgreSQL

Question

It is currently in a monthly report. I want it to have 4 quarters Q1, Q2, Q3, Q4 instead of Jan, Feb, Mar, Apr, May, Jun, Jul, etc.

Here is the code:

create table sales(year int, month int, qty int);
insert into sales values(2007, 1, 1000);
insert into sales values(2007, 2, 1500);
insert into sales values(2007, 7, 500);
insert into sales values(2007, 11, 1500);
insert into sales values(2007, 12, 2000);
insert into sales values(2008, 1, 1000);

select * from crosstab(
  'select year, month, qty from sales order by 1',
  'select m from generate_series(1,12) m'
) as (
  year int,
  "Jan" int,
  "Feb" int,
  "Mar" int,
  "Apr" int,
  "May" int,
  "Jun" int,
  "Jul" int,
  "Aug" int,
  "Sep" int,
  "Oct" int,
  "Nov" int,
  "Dec" int
);

Answer 1

I would just use conditional aggregation:

select year,
       sum(qty) filter (where month between 1 and 3) as q1,
       sum(qty) filter (where month between 4 and 6) as q2,
       sum(qty) filter (where month between 7 and 9) as q3,
       sum(qty) filter (where month between 10 and 12) as q4
from t
group by year;