试试这个查询：
SELECT bl.user_id, SUM( ph.amount ) PAIDOUT
FROM (
SELECT distinct blocks.user_id
FROM blocks
WHERE confirms > 520
) bl
LEFT JOIN payout_history ph
ON bl.user_id = ph.user_id
GROUP BY ph.user_id
;
SQLFiddle --> http://sqlfiddle.com/#!2/7b988/48
--- 编辑 --- 解释查询是如何工作的（或者更确切地说，为什么你的查询不起作用）----
查看预期结果，查询似乎应该计算amount每个列的总和user_id，但仅针对那些user_id也在blocks表中且blocks.confirms值大于 520
的列。简单的连接（也是左外连接）不能工作在这种情况下，因为blocks表可以包含许多相同的记录user_id，例如返回行的查询仅user_id=110给出以下结果：
SELECT *
FROM blocks
WHERE confirms > 520
AND user_id = 110;
+ ------- + ------------ + ----------- + ------------- +
| id | user_id | reward | confirms |
+ ------- + ------------ + ----------- + ------------- +
| 0 | 110 | 20.89832115 | 521 |
| 65174 | 110 | 3.80357075 | 698 |
| 65204 | 110 | 4.41933060 | 668 |
| 65218 | 110 | 4.69059801 | 654 |
| 65219 | 110 | 4.70222521 | 653 |
| 65230 | 110 | 4.82805490 | 642 |
| 65265 | 110 | 5.25058079 | 607 |
| 65316 | 110 | 6.17262650 | 556 |
+ ------- + ------------ + ----------- + ------------- +
直线连接（和左/右外连接）以这种方式工作，它从第一个连接表中获取每条记录，并将这条记录与另一个连接表中的所有行配对（组合）以满足连接条件。
在我们的例子中，左连接产生以下结果集：
SELECT *
FROM blocks
LEFT JOIN payout_history
ON blocks.user_id = payout_history.user_id
WHERE confirms > 520
AND blocks.user_id = 110;
+ ------- + ------- + ----------- + -------- + --- + ------- + ----------- +
| id | user_id | reward | confirms | id | user_id | amount |
+ ------- + ------- + ----------- + -------- + --- + ------- + ----------- +
| 0 | 110 | 20.89832115 | 521 | 1 | 110 | 20.898319 |
| 65174 | 110 | 3.80357075 | 698 | 1 | 110 | 20.898319 |
| 65204 | 110 | 4.41933060 | 668 | 1 | 110 | 20.898319 |
| 65218 | 110 | 4.69059801 | 654 | 1 | 110 | 20.898319 |
| 65219 | 110 | 4.70222521 | 653 | 1 | 110 | 20.898319 |
| 65230 | 110 | 4.82805490 | 642 | 1 | 110 | 20.898319 |
| 65265 | 110 | 5.25058079 | 607 | 1 | 110 | 20.898319 |
| 65316 | 110 | 6.17262650 | 556 | 1 | 110 | 20.898319 |
+ ------- + ------- + ----------- + -------- + --- + ------- + ----------- +
现在如果我们添加SUM( amount ) .... GROUP BY user_id，MySql 将计算amount上述结果集中所有值的总和（8 行 * 20.898 = ~ 167.184）
SELECT blocks.user_id, sum( amount)
FROM blocks
LEFT JOIN payout_history
ON blocks.user_id = payout_history.user_id
WHERE confirms > 520
AND blocks.user_id = 110
GROUP BY blocks.user_id;
+ ------------ + ----------------- +
| user_id | sum( amount) |
+ ------------ + ----------------- +
| 110 | 167.186554 |
+ ------------ + ----------------- +
正如你在这种情况下看到的，连接并没有给我们想要的结果——我们需要一些命名的东西a semi join——下面是半连接的不同变体，试一试：
SELECT bl.user_id, SUM( ph.amount ) PAIDOUT
FROM (
SELECT distinct blocks.user_id
FROM blocks
WHERE confirms > 520
) bl
LEFT JOIN payout_history ph
ON bl.user_id = ph.user_id
GROUP BY ph.user_id
;
SELECT ph.user_id, SUM( ph.amount ) PAIDOUT
FROM payout_history ph
WHERE ph.user_id IN (
SELECT user_id FROM blocks
WHERE confirms > 520
)
GROUP BY ph.user_id
;
SELECT ph.user_id, SUM( ph.amount ) PAIDOUT
FROM payout_history ph
WHERE EXISTS (
SELECT 1 FROM blocks bl
WHERE bl.user_id = ph.user_id
AND bl.confirms > 520
)
GROUP BY ph.user_id
;