获取2个不同相关表中两个字段的总和

Question

I'd like to get the SUM of the amount column in two related tables.

Invoices Table:

-----------------------------------------
| id  | student_id  | created | updated |
-----------------------------------------
|  5  |     25      | date    | date    |
-----------------------------------------

Invoice Items Table:

------------------------------
| id  | invoice_id  | amount |
------------------------------
|  1  |     5       | 250    |
------------------------------
|  2  |     5       | 100    |
------------------------------
|  3  |     5       | 40     |
------------------------------

Payments Table:

------------------------------
| id  | invoice_id  | amount |
------------------------------
|  1  |     5       | 100    |
------------------------------
|  2  |     5       | 290    |
------------------------------

Desired Output:

--------------------------------------
| id  | invoiceTotal  | paymentTotal |
--------------------------------------
|  1  |     390       |    390       |
--------------------------------------

The query I've tried

SELECT 
    i.id,
    sum(ii.amount) as invoiceTotal,
    sum(p.amount) as paymentTotal
FROM 
    invoices i
LEFT JOIN 
    invoice_items ii ON i.id = ii.invoice_id
LEFT JOIN 
    payments p ON i.id = p.invoice_id
WHERE 
    i.student_id = '25'
GROUP BY 
    i.id

What this seems to do is calculate the sum of the payments properly but the invoice_items.amount appears to have been duplicated by 6 (which is the number of payments there are).

I have read similar questions on SO here and here but the examples are so much more complex than what I'm trying to do and I can't figure out what to put where.

Answer 1

The join causes a problem with cartesian products. If a student has multiple invoice items and payments, then the totals will be wrong.

One approach that works best for all invoices is a union all / group by approach:

select i.id, sum(invoiceTotal) as invoiceTotal, sum(paymentTotal) as paymentTotal
from ((select i.id, 0 as invoiceTotal, 0 as paymentTotal
       from invoices i
      ) union all
      (select ii.invoiceId, sum(ii.amount) as invoiceTotal, NULL
       from invoiceitems ii
       group by ii.invoiceId
      ) union all
      (select p.invoiceId, 0, sum(p.amount) as paymentTotal
       from payments p
       group by p.invoiceId
      )
     ) iip
group by id;

For a single student, I would recommend correlated subqueries:

select i.id,
       (select sum(ii.amount)
        from invoiceitems ii
        where ii.invoiceid = i.id
       ) as totalAmount,
       (select sum(p.amount)
        from payment p
        where p.invoiceid = i.id
       ) as paymentAmount
from invoices i
where i.studentid = 25;