Question

我想在两个相关表格中获得amount列的SUM。

发票表：

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

发票项目表：

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

付款表

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

期望输出：

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

我尝试的查询

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

这似乎做的是正确计算付款总额，但invoice_items.amount似乎已被重复6（这是payments的数量。）

我已经在SO here和here上阅读了类似的问题，但是这些例子比我想做的要复杂得多，我无法弄清楚要放在哪里。< / p>

Answer 1

联接导致笛卡儿产品出现问题。如果学生有多个发票项目和付款，那么总计将是错误的。

最适合所有发票的一种方法是union all / group by方法：

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;

对于单个学生，我会推荐相关的子查询：

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;

获取2个不同相关表中两个字段的SUM

1 个答案: