Issues with postgreSQL subqueries - sql

Question

I have the following chunk of code, in which in trying to count the sales of beef, chicken and pork in each month of the last year (i also need to determine the market share of the meats each month)
SELECT
CAST(EXTRACT('MONTH' FROM TO_TIMESTAMP(FULLDATE, 'YYYY-MM-DD')) AS INT) AS month
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
(
SELECT
COUNT (purchaseid) AS total_sales
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
WHERE category = 'whole milk' OR category = 'yogurt' OR category = 'domestic eggs'
GROUP BY month
) a
GROUP BY month
ORDER BY month
The expected result is the following image
EDIT to add the exact error message
but in getting this error message
syntax error at or near "SELECT"
LINE 6: SELECT
^
[SQL: SELECT
CAST(EXTRACT('MONTH' FROM TO_TIMESTAMP(FULLDATE, 'YYYY-MM-DD')) AS INT) AS month
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
(
SELECT
COUNT (purchaseid) AS total_sales
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
WHERE category = 'whole milk' OR category = 'yogurt' OR category = 'domestic eggs'
GROUP BY month
) a
GROUP BY month
ORDER BY month
This is the data schema i'm working with.
EDIT
I'm aware i can query the total_sales like this:
SELECT
CAST(EXTRACT('MONTH' FROM TO_TIMESTAMP(FULLDATE, 'YYYY-MM-DD')) AS INT) AS month,
COUNT (purchaseid) AS total_sales
FROM purchases_2020
JOIN categories ON purchases_2020.purchaseid = categories.purchase_id
WHERE category = 'beef' OR category = 'pork' OR category = 'chicken'
GROUP BY month
ORDER BY month
But doing it like this locks me out of doing of writting the market_share formula on the select statement because of the WHERE statement no being inside a subquery.

Answer 1

This query should give you the count of sales by month and category. I can't test it because I don't have datas.
SELECT
c.category,
EXTRACT('MONTH' FROM FULLDATE) AS month,
count(purchaseid) AS total_sales
FROM purchases_2020 p JOIN categories c ON p.purchaseid = c.purchase_id
WHERE category in ('beef','pork','chicken')
GROUP BY month,c.category
ORDER BY month,c.category;