SQL is it possible to have multiple subqueries in From clause? - sql

Question

So I have 2 tables.
the first one would be the bill:
id
total
client_code
created_at
1
10
1
2022-02-01
2
20
1
2022-02-01
3
20
3
2022-03-01
the second would be the product (for a bill):
bill_id
category
total
1
Electronic
2
1
Food
5
1
Food
3
2
Food
10
2
Food
10
3
Food
10
3
Food
10
What I want to get with my query is the average spending by a client for each month over the last 4 months for example.
my query right now is looking like this for the first 2 months and it works, I get the result I want:
SELECT
COALESCE(AVG(first.total),0),
COALESCE(AVG(second.total),0)
FROM
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-02-01' AND b.created_at < '2022-03-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as first),
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-02-01' AND b.created_at < '2022-03-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as second)
but as soon as I get to 3 months it doesn't anymore:
SELECT
COALESCE(AVG(first.total),0),
COALESCE(AVG(second.total),0),
COALESCE(AVG(third.total),0)
FROM
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-02-01' AND b.created_at < '2022-03-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as first),
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-02-01' AND b.created_at < '2022-03-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as second),
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-03-01' AND b.created_at < '2022-01-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as three)
Is there a rule that keeps me from having more than two subqueries inside my from clause?
If so is there an other way of doing this? My real problem is for 12 month actually not 4. I already have a working solution but performance wise it's bad, that is why I am trying this.
My working solution is looking like that:
(SELECT
COALESCE(AVG(req.total),0)
FROM
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-02-01' AND b.created_at < '2022-03-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as req)
UNION ALL
(SELECT
COALESCE(AVG(req.total),0)
FROM
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-03-01' AND b.created_at < '2022-04-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as req)
UNION ALL
(SELECT
COALESCE(AVG(req.total),0)
FROM
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-04-01' AND b.created_at < '2022-05-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as req)
UNION ALL
(SELECT
COALESCE(AVG(req.total),0)
FROM
(SELECT
SUM(p.total) as total
FROM
bill b
INNER JOIN product p one b.id = p.bill_id
WHERE
b.created_at >= '2022-05-01' AND b.created_at < '2022-06-01' AND
p.category = 'FOOD'
GROUP BY
p.client_code) as req)

Answer 1

Something like this (but I'm pretty sure I don't have the right way of getting months, years in postgres - you will have to fix that!)
SELECT
A.client_code,
avg(A.total) as AvgMonthlyTotal
FROM
(
SELECT
SUM(p.total) as total,
month(b.created_at),
p.client_code
FROM
bill b
INNER JOIN product p
on b.id = p.bill_id
WHERE
p.Category = 'FOOD'
and year(b.created_at) = 2022
GROUP BY
month(b.created_at)
p.client_code
) A
GROUP BY A.client_code;