Combine different results of "group by" queries in the same table - sql

Question

I need to make some comparation between 2 years: sales by product, sales by category, etc.
How can I have this in one table having 3 columns:
first column = product, category, etc
second column = sales in 2021
third column = sales in 2022
Sample of queries that must be combined in one single table as the one below
select product_code, sum(amount)
from product
where year = '2021'
group by product_code
select product_code, sum(amount)
from product
where year = '2022'
group by product_code
select category_code, sum(amount)
from category
where year = '2021'
group by category_code
select category_code, sum(amount)
from category
where year = '2022'
group by category_code
Please, see the final table
[1]: https://i.stack.imgur.com/smF7h.png
NOTE!
If for instance in 2021 there was no "product D", it will be 0 for "Sales_2021" or the "product A" is no longer present in 2022, it will be 0 for "Sales_2022".
Thank you

Answer 1

You need two things here:
Conditional aggregation (a CASE expression inside the aggregation function) in order to get 2021 and 2022 in one go.
A union of two intermediate result sets (product figures UNION ALL category figures).
And as any table - and a query result is again a table - is unordered, we need an ORDER BY at last to get products first and categories second and also the products ordered alphabetically and the categories, too.
The complete query:
select category_or_product, sales_2021, sales_2022
from
(
select
product_code as category_or_product,
sum(case when year = 2021 then amount else 0 end) as sales_2021,
sum(case when year = 2021 then amount else 0 end) as sales_2022,
1 as product_first
from product
group by product_code
union all
select
category_code as category_or_product,
sum(case when year = 2021 then amount else 0 end) as sales_2021,
sum(case when year = 2021 then amount else 0 end) as sales_2022,
2 as product_first
from category
group by category_code
) unioned
order by product_first, category_or_product;