Decluttering a SQL query - sql

Question

For a practice project i wrote the following query and i was wondering if there is way to make it more efficient than writing everything 12 times like a for loop for sql.
CREATE TABLE temp (month INT, total_sales INT, market_share decimal(5,2), year_change decimal(5,2))
insert into temp (month)
Values (1)
UPDATE temp
SET total_sales = (
SELECT COUNT(purchases_2020.purchaseid)
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
WHERE (categories.category = 'whole milk' OR categories.category = 'yogurt' OR categories.category = 'domestic eggs') AND (purchases_2020.fulldate BETWEEN '2020-01-01' AND '2020-01-31')
)
WHERE month = 1
UPDATE temp
SET market_share = (
SELECT (SELECT 100 * COUNT(purchases_2020.purchaseid)
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
WHERE (categories.category = 'whole milk' OR categories.category = 'yogurt' OR categories.category = 'domestic eggs') AND (purchases_2020.fulldate BETWEEN '2020-01-01' AND '2020-01-31'))
* 1. /
(SELECT COUNT(purchases_2020.purchaseid)
FROM purchases_2020
WHERE purchases_2020.fulldate BETWEEN '2020-01-01' AND '2020-01-31')
)
WHERE month = 1
UPDATE temp
SET year_change = (
SELECT market_share -
(SELECT
(SELECT 100 * COUNT(purchases_2019.purchase_id)
FROM purchases_2019
JOIN categories ON purchases_2019.purchase_id = categories.purchase_id
WHERE (categories.category = 'whole milk' OR categories.category = 'yogurt' OR categories.category = 'domestic eggs') AND (purchases_2019.full_date BETWEEN '2019-01-01' AND '2019-01-31'))
* 1./
(SELECT COUNT(purchases_2019.purchase_id)
FROM purchases_2019
WHERE purchases_2019.full_date BETWEEN '2019-01-01' AND '2019-01-31'))
FROM temp
WHERE month = 1
)
WHERE month = 1
EDIT
I was given the 3 tables represented on the following database schema , and im trying to create a table with the total sales of dairy every month, the monthly market share of the dairy products and the difference between the 2020 monthly market share and the 2019 monthly market share (the year change colunm)
There is also an aritmethic error somewhere, when checking the project i get the following message ResultSet does not contain the correct numeric values! and im at my wits end looking for it butmy priority is to decluter the query.

Answer 1

Your error message tells me that you are trying to run this from a reporting tool or a host language.
It also makes no sense to put the data into separate tables by years.
SQL is a declarative language that works with data as sets.
Instead of pushing the results into table temp, try writing a query like this:
with all_data as (
select p.fulldate, p.purchaseid, c.category,
extract(year from p.fulldate) as year,
extract(month from p.fulldate) as month
from purchases_2020 p
join categories c on c.purchase_id = p.purchaseid
union all
select p.fulldate, p.purchaseid, c.category,
extract(year from p.fulldate) as year,
extract(month from p.fulldate) as month
from purchases_2019 p
join categories c on c.purchase_id = p.purchaseid
), kpis as (
select year, month,
count(purchaseid)
filter (where category in ('whole milk', 'yogurt', 'domestic eggs'))
as dairy_sales,
count(purchaseid) * 1.0 as total_sales
from all_data
group by year, month
)
select ty.month, ty.dairy_sales as total_sales,
100.0 * ty.dairy_sales / ty.total_sales as market_share,
100.0 * ( (ty.dairy_sales / ty.total_sales)
- (ly.dairy_sales / ly.total_sales)) as year_change
from kpis ty
join kpis ly
on (ly.year, ly.month) = (ty.year - 1, ty.month);