How to calculate current month / six months ago and result as a percent change in Postgresql?

Question

create table your_table(type text,compdate date,amount numeric);
insert into your_table values
('A','2022-01-01',50),
('A','2022-02-01',76),
('A','2022-03-01',300),
('A','2022-04-01',234),
('A','2022-05-01',14),
('A','2022-06-01',9),
  
('B','2022-01-01',201),
('B','2022-02-01',33),
('B','2022-03-01',90),
('B','2022-04-01',41),
('B','2022-05-01',11),
('B','2022-06-01',5),
  
('C','2022-01-01',573),
('C','2022-02-01',77),
('C','2022-03-01',109),
('C','2022-04-01',137),
('C','2022-05-01',405),
('C','2022-06-01',621);

I am trying to calculate to show the percentage change in $ from 6 months prior to today's date for each type. In example:

• Type A decreased -82% over six months.
• Type B decreased -97.5%
• Type C increased +8.4%.

How do I write this in postgresql mixed in with other statements?

Answer 1

It looks like comparing against 5, not 6 months prior, and 2022-06-01 isn't today's date.

Join the table with itself based on the matching type and desired time difference. Demo

select 
  b.type,
  b.compdate,
  a.compdate "6 months earlier",
  b.amount "amount 6 months back",
  round(-(100-b.amount/a.amount*100),2) "change"
from your_table a 
    inner join your_table b 
        on a.type=b.type 
        and a.compdate = b.compdate - '5 months'::interval;
    

-- type |  compdate  | 6 months earlier | amount 6 months back | change
--------+------------+------------------+----------------------+--------
-- A    | 2022-06-01 | 2022-01-01       |                    9 | -82.00
-- B    | 2022-06-01 | 2022-01-01       |                    5 | -97.51
-- C    | 2022-06-01 | 2022-01-01       |                  621 |   8.38