在MySQL中选择分组

Question

我有两张桌子：

'支付'

|id_tax|value|id_currency|
--------------------------
|1     |100  |12         |
|1     |200  |12         |
|1     |300  |13         |
|2     |200  |12         |
|2     |200  |12         |
--------------------------

'速率'

|id_currency|rate|
------------------
|12         |1   |
|13         |6,12|
------------------

这将导致什么SQL查询？

|id_tax|value               |
-----------------------------
|1     |100*1+200*1+300*6,12|
|2     |200*1+200*1         |
-----------------------------

Answer 1

SELECT  id_tax, SUM(value * rate)
FROM    pay p
JOIN    rate r
ON      r.id_currency = p.id_currency
GROUP BY
        id_tax

Answer 2

在MySQL中，您可以使用会话变量计算它：

SET @s = 0; 

SELECT  id_tax, @s := s +(value * rate)
FROM    pay p
JOIN    rate r
ON      r.id_currency = p.id_currency
GROUP BY
   p.id_tax

或以纯粹基于集合但效率较低的方式：

SELECT  pay.id_currency, pay.val, SUM(pay.value * rate.rate)
FROM    pay
JOIN    rate
ON      rate.id_currency = pay.id_currency
GROUP BY
        pay.id_tax