Create a pivot table for Month over Month variation - postgresql

Question

I have these records returned from a query
+---------+--------------+-----------+----------+
| Country | other fields | sales | date |
+---------+--------------+-----------+----------+
| US | 1 | $100.00 | 01/01/21 |
| CA | 1 | $100.00 | 01/01/21 |
| UK | 1 | $100.00 | 01/01/21 |
| FR | 1 | $100.00 | 01/01/21 |
| US | 1 | $200.00 | 01/02/21 |
| CA | 1 | $200.00 | 01/02/21 |
| UK | 1 | $200.00 | 01/02/21 |
| FR | 1 | $200.00 | 01/02/21 |
And I want to show the sales variation from one month to previous, like this:
| Country | 01/02/21 | 01/01/21 | Var% |
| US | $200.00 | $100.00 | 100% |
| CA | $200.00 | $100.00 | 100% |
| FR | $200.00 | $100.00 | 100% |
+---------+--------------+-----------+----------+
How could be done with a Postgres query?

Answer 1

if you always comparing two month only :
select country
, sum(sales) filter (where date ='01/01/21') month1
, sum(sales) filter (where date ='01/02/21') month2
, ((sum(sales) filter (where date ='01/02/21') /sum(sales) filter (where date ='01/01/21')) - 1) * 100 var
from tablename
where date in ('01/01/21' , '01/02/21')
group by country
you also can look at crosstab from tablefunc extension which basically does the same as above query.
CREATE EXTENSION IF NOT EXISTS tablefunc;
select * ,("01/02/21" /"01/01/21") - 1) * 100 var
from(
select * from crosstab ('select Country,date , sales from tablename')
as ct(country varchar(2),"01/01/21" money , "01/02/21" money)
) t
for more info about crosstab , see tablefunc
but if you want to show date in rows instead of columns, you can easily generalize it for all the dates :
select *
, ((sales / LAG(sales,1,1) over (partition by country order by date)) -1)* 100 var
from
country