INNER JOIN change SUM result - postgresql

Question

CREATE TABLE beauty.customer_payments
(
customer_id integer,
date date,
amount numeric(10,2),
CONSTRAINT customer_payments_customer_id_fkey FOREIGN KEY (customer_id)
REFERENCES beauty.customers (customer_id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE NO ACTION
)
CREATE TABLE beauty.sales
(
product_id integer,
customer_id integer,
sell_date date NOT NULL,
qty integer NOT NULL,
sell_price numeric(10,2) NOT NULL,
expiry_date date NOT NULL,
CONSTRAINT sales_customer_id_fkey FOREIGN KEY (customer_id)
REFERENCES beauty.customers (customer_id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE NO ACTION,
CONSTRAINT sales_product_id_fkey FOREIGN KEY (product_id)
REFERENCES beauty.products (product_id) MATCH SIMPLE
ON UPDATE NO ACTION
ON DELETE NO ACTION
)
Balance of payments beauty.customer_payments for customer_id=6 is 0
SELECT * FROM beauty.customer_payments
WHERE customer_id=6;
customer_id
date
amount
6
2020-11-14
75.00
6
2020-11-14
-75.00
SELECT * FROM beauty.sales
WHERE customer_id=6;
product_id
customer_id
sell_date
qty
sell_price
expiry_date
76
6
2020-11-14
1
75.00
2022-03-03
83
6
2020-11-14
1
10.00
2022-06-23
85
6
2020-11-14
1
10.00
2022-06-23
44
6
2020-11-14
1
12.00
2022-06-23
41
6
2020-11-14
1
15.00
2022-03-26
96
6
2020-11-14
1
75.00
2022-03-15
28
6
2020-11-14
1
4.00
2022-01-22
33
6
2020-11-14
1
4.00
2023-01-23
37
6
2020-11-14
1
4.00
2023-01-23
40
6
2020-11-14
1
4.00
2023-08-13
(10 rows)
SELECT customer_id, SUM(qty * sell_price) AS purchased
FROM beauty.sales
WHERE customer_id=6
GROUP BY customer_id;
customer_id
purchased
6
213.00
SELECT s.customer_id,
SUM(qty * sell_price) AS purchased,
SUM(cp.amount) AS paid,
SUM(qty * sell_price - cp.amount) AS balance
FROM beauty.sales s
INNER JOIN beauty.customer_payments cp
ON cp.customer_id = s.customer_id
WHERE s.customer_id=6
GROUP BY s.customer_id;
customer_id
purchased
paid
balance
6
1065.00
0.00
1065.00
Please advise WHY after adding JOIN (INNER, LEFT, RIGHT) calculation goes wrong and how to solve this multi-calculation issue?
As I see similar question all of them based on not cross-tables calculations like SUM(qty * sell_price - cp.amount)
Delete payments
`
DELETE FROM beauty.customer_payments
WHERE customer_id=6;`
Add new ZERO payment
INSERT INTO beauty.customer_payments(
customer_id, date, amount)
VALUES (6, '2020-11-17', 0);
customer_id
purchased
paid
balance
6
213.00
0.00
213.00
Add payment 10
INSERT INTO beauty.customer_payments(
customer_id, date, amount)
VALUES (6, '2020-11-17', 10);
SELECT * FROM beauty.customer_payments WHERE customer_id=6;
customer_id
date
amount
6
2020-11-17
0.00
6
2020-11-17
10.00
SELECT s.customer_id,
.......
INNER JOIN beauty.customer_payments cp
.......
customer_id
purchased
paid
balance
6
426.00
100.00
326.00
Correct payment with negative amount
INSERT INTO beauty.customer_payments(
customer_id, date, amount)
VALUES (6, '2020-11-17', -10);
SELECT * FROM beauty.customer_payments WHERE customer_id=6;
customer_id
date
amount
6
2020-11-17
0.00
6
2020-11-17
10.00
6
2020-11-17
-10.00
SELECT s.customer_id,
.......
INNER JOIN beauty.customer_payments cp
.......
customer_id
purchased
paid
balance
6
639.00
0.00
639.00
What is this `INNER JOIN' calculate?

Answer 1

In general, you probably wouldn't want to match each payment to each sale (unless there's an additional identifier matching specific payments to specific sales, not just matching each to a customer).
If a customer has 2 sales for $10 and $15, and two payments for $9 and $14, your join is going to match each payment to each sale for that customer, creating something like
Sale
Payment
$10
$9
$10
$14
$15
$9
$15
$14
So the sum of the sales after the join will be $50, not $25 (as you might be expecting). I think the above answers your question about why the join doesn't do what you expect.
The exact query you want might be a little different (do you want all customers even if they have no sales? is it possible for customers to have payments if they don't have a sale?), but in general I'd expect something like the following to work. There are multiple ways of doing this, but I think the following is easy to understand since it aggregates the data into one payment row per customer and one sales row per customer before joining them.
SELECT
s.customer_id,
s.purchased,
cp.amount as paid,
s.purchased - cp.amount as balance
FROM
(SELECT s.customer_id,
SUM(s.qty * s.sell_price) AS purchased
FROM beauty.sales s
GROUP BY s.customer_id) s
LEFT OUTER JOIN
(SELECT cp.customer_id,
SUM(cp.amount) AS amount
FROM beauty.customer_payments cp
GROUP BY cp.customer_id) cp
ON s.customer_id = cp.customer_id
WHERE s.customer_id = 6