MySQL, Join two tables with one table? - mysql

Question

I have one table customer, and one bill, and one sell.
Customer table
id-----name
Bill table
id-----customer_id
Sell table
id-----customer_id-----bill_id-----qtt-----price
A customer can have the records in the sell table with customer_id, and also have the bill record in the bill table with customer_id and this bill record has record in the sell table with bill_id.
This means a customer can have direct or indirect (in this case by passing the bill table) with sell table.
Now how to join tables that retrieve the total sell of a customer with ascending or deciding order?
Any idea?
I have tried many ways for example something like below, but none of them was working:
SELECT
sell.id AS sell_id,
customer.id,
bill.id AS bill_id,
customer.`name`,
sell.quantity*sell.price AS sell_price
FROM
customer_tb customer
JOIN bill ON bill.customer_id = customer.id
JOIN sell ON sell.customer_id = customer.id OR sell.bill_id = bill.id
NOTE: In case of bill table has a record in the sell table, the customer_id column is NULL and also same for the customer sell record the bill_id is NULL, this means in the sell table in the same entry only one of the (customer_id, bill_id) column has value.

Answer 1

You have the customer column in your sell tables, you don't need to join with bill table.
SELECT
customer.id,
customer.name,
SUM(sell.quantity * sell.price) AS total_amount
FROM customer_tb as customer
INNER JOIN sell
On sell.customer_id = customer.id
GROUP BY customer.id
You have to group your rows by the customer to get their sum independently

Answer 2

Your challenge is to look up the correct Customer.name for each Sell row.
You need to join Sell to Customer, and also join Sell to Bill to Customer.
How to do this?
FROM Sell s
LEFT JOIN Cust c1 ON s.customer_id = c1.id
LEFT JOIN Bill b on s.bill_id = b.id
LEFT JOIN Cust c2 ON b.customer_id = c2.id
That gives us:
s.id: the Sale id
s.quantity, s.price: the business details of the Sale
c1.name: the Customer name directly from the Sale's customer_id
c2.name: the Customer name indirectly through the Sale's bill_id
Because we use LEFT JOINs, either name can be NULL depending on which ids are null. And, I believe you favor using the direct name over the indirect name if you have both.
That means we use COALESCE() to choose one name from the two. Something like this
SELECT s.id, s.Quantity, s.Price,
COALESCE (c1.name, c2.name) name,
COALESCE (c1.id, c2.id) customer_id
FROM Sell s
LEFT JOIN Cust c1 ON s.customer_id = c1.id
LEFT JOIN Bill b on s.bill_id = b.id
LEFT JOIN Cust c2 ON b.customer_id = c2.id
But, if you have a chance to rework this database before you put a ton of data into it, please consider redesigning things to get rid of this ambiguity. It's going to drive you mad and cost you bigger servers if you have to make it work well with megarows.