Question

假设我有两个表：

客户

  Name   |   id   |
-------------------
  Benny  |    1   |
  Wilson |    2   |
  Joe    |    3   |
  Austin |    4   |

订单

  Product  |   id   |
---------------------
  TV       |    1   |
  Hifi-set |    1   |
  HTPC     |    1   |
  CD       |    1   |
  DVD      |    1   |
  CD       |    1   |
  DVD      |    1   |

这就是我想要的结果：

  Name   | Orders |
-------------------
  Benny  |    7   |
  Wilson |    0   |
  Joe    |    0   |
  Austin |    0   |

我不熟悉SQL，但我尝试过：

SELECT c.Name FROM Customers AS c LEFT JOIN Orders AS o ON c.id=o.id GROUP BY c.Name

但得到了错误的结果：

  Name   | Orders |
-------------------
  Benny  |    4   |
  Wilson |    1   |
  Joe    |    1   |
  Austin |    1   |

我该怎么办？

Answer 1

尝试：

select 
   c.Name, 
   (select count(1) from Orders where ID=c.ID) 
from 
   Customers as c

Answer 2

如果不使用SubQuery，您也可以改为使用JOIN。

SELECT   a.Name, COUNT(b.id)
FROM     Customers a LEFT JOIN Orders b
            on a.ID = b.ID
GROUP BY a.Name

如何选择表并计算其他表

2 个答案: