mysql - MySQL的左联接与COUNT

我有两张桌子，顾客和售货员。我想计算每个客户的销售额，并为每个商店创建每月销售额表。
我想制作一些类似的东西；

------------------------------
month  |  customers  | sales  |
------------------------------
1/2013 |      5      |   2    |
2/2013 |      21     |   9    |
3/2013 |      14     |   4    |
4/2013 |      9      |   3    |

但我在使用以下方法时，无法使销售计数正确无误；

SELECT CONCAT(MONTH(c.added), '/', YEAR(c.added)), count(c.id), count(s.id)
FROM customers c
LEFT JOIN sales s
ON s.customer_id = c.id AND MONTH(c.added) = MONTH(s.added) AND YEAR(c.added) = YEAR(s.added)
WHERE c.store_id = 1
GROUP BY YEAR(c.added), MONTH(c.added);

客户表；

-------------------------------
id    |   store_id  | added    |
-------------------------------
1     |      1      |2013-02-01 |
2     |      1      |2013-02-02 |
3     |      1      |2013-03-16 |

销售表；

---------------------------------
id    |   added    | customer_id |
---------------------------------
1     | 2013-02-18 |     3       |
2     | 2013-03-02 |     2       |
3     | 2013-03-16 |     3       |

有人能帮忙吗？
谢谢

最佳答案

（更新）现有的查询只计算客户添加的同一个月的销售额。改为试试这个：

SELECT CONCAT(MONTH(sq.added), '/', YEAR(sq.added)) month_year,
       sum(sq.customer_count),
       sum(sq.sales_count)
FROM (select s.added, 0 customer_count, 1 sales_count
      from customers c
      JOIN sales s ON s.customer_id = c.id
      WHERE c.store_id = 1
      union all
      select added, 1 customer_count, 0 sales_count
      from customers
      WHERE store_id = 1) sq
GROUP BY YEAR(sq.added), MONTH(sq.added);

关于mysql - MySQL的左联接与COUNT，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/16917167/