这个问题在这里已经有答案了：

同一列的两个where条件使用group by（2个答案）
两年前关门了。
我有一张t2桌。我需要 vqty 分组依据 from_br 以及 to_br 作为两列。我的table在下面。我的数据库架构在SQLFIDLE中。
t1级：

id    branch_name
-------------------
1     branch1
2     branch2
3     branch3
4     branch4
5     branch5
6     branch6

t2段：

id    from_br    to_br     qty
--------------------------------
1     1         3          10
2     2         4          20
3     3         2          30
4     2         3          40
5     1         4          50 
6     4         5          60

期望结果：

branch_name    send        received
-------------------------------
branch1        60          0          
branch2        60          30
branch3        30          50
branch4        60          70
branch5        0           60

我尝试的查询是：

SELECT t1.branch_name, 
   COALESCE(SUM(send.vqty), 0) AS send, 
   COALESCE(SUM(receive.vqty), 0) AS received 
FROM t1  
LEFT JOIN t2 AS send on t1.id = send.from_br  
LEFT JOIN t2 AS receive on t1.id = receive.to_br 
GROUP BY t1.branch_name, send.from_br, receive.from_br