MySQL SUM function in multiple joins

Question

Hi so this is my case I have those tables

Customer {id,name}
Charges {id,amount,customer_id}
Taxes {id,amount,charge_id}

so I want to SUM amoun

Answer 1

You want to know if you can do this without subqueries. No, you can't.

If a row in Charges has more than one corresponding row in Taxes, you can't simply join the tables without duplicating Charges rows. Then, as you have discovered, when you sum them up, you'll get multiple copies.

You need a way to get a virtual table (a subquery) with one row for each Charge.

                    SELECT ch.customer_id,
                           ch.amount amount,
                           tx.tax tax
                      FROM Charges
                      LEFT JOIN (  
                                  SELECT SUM(amount) tax,
                                         charge_id 
                                    FROM Taxes
                                   GROUP BY charge_id
                          ) tx ON ch.id = tx.charge_id

You can then join that subquery to your Customer table to summarize sales by customer.