Question

我试图弄清楚为什么联接会带来它所带来的结果。

这是一个简单的测试我正在努力。

我有两个表：Table1和Table2。

表1

-------------------
id    name      amount
---------------------
1   | Fred      | 2
3   | Fred      | 3
5   | Fred      | 4
2   | Hellen    | 3
4   | Hellen    | 3
6   | Hellen    | 3

表2

-------------------
id    name      amount
---------------------
1   | Fred      |4
2   | Hellen    |3
3   | Paul      |3
4   | Fridah    |5

当我使用以下声明时

SELECT p.name,sum(p.amount) as amount FROM `table1` as p  
left join table2 as c on p.name = c.name GROUP BY c.name , p.name

我得到了这个结果

---------------
name    amount
---------------     
Fridah  | 5
Paul    | 3
Fred    | 12
Hellen  | 9

但这真的很奇怪。我想从table1中得到amount的总和，但是联接给了我两个表的总和。为什么是这样？

Answer 1

更改group by子句，仅包含p.name GROUP BY p.name，如下所示。在这里查看演示小提琴http://sqlfiddle.com/#!2/6c0e3/1

SELECT p.name,sum(p.amount) as amount 
FROM `table1` as p  
left join table2 as c on p.name = c.name 
GROUP BY p.name

结果

NAME    AMOUNT
Fred    9
Hellen  9

修改

您可以单独计算总和，然后加入派生表，以便从两个表中获得正确的总和，如下所示。请参阅更新小提琴Here

select X.name,X.amount,Y.amount2 from ( select name,sum(amount) as amount from table1 group by name ) X left join ( select name,sum(amount) as amount2 from table2 group by name ) Y on X.name = Y.name

结果：

NAME AMOUNT AMOUNT2 Fred 9 4 Hellen 9 3

Answer 2

这是您的查询：

SELECT p.name, sum(p.amount) as amount
FROM `table1` as p left join
     table2 as c
     on p.name = c.name
GROUP BY c.name , p.name;

您的结果表明您正在运行的查询是：

SELECT p.name, sum(c.amount) as amount
-------------------^
FROM `table1` as p left join
     table2 as c
     on p.name = c.name
GROUP BY c.name , p.name;

如果您运行正确的查询，您将获得预期的结果。

编辑：

正确的查询如下：

SELECT c.name, sum(p.amount) as amount
FROM table2 as c left join
     table1 as p
     on p.name = c.name
GROUP BY c.name;

Answer 3

我找到了答案。通过向求和的列添加不同，我能够得到正确的结果。看看http://sqlfiddle.com/#!2/6c0e3/12/0

谢谢大家的帮助。

MYSQL LEFT加入带来错误的结果

3 个答案: