sql - 在三个表上使用 count() 进行内连接

Question

简单而快速的问题，我有这些表:

//table people
| pe_id | pe_name |
| 1  | Foo  |
| 2  | Bar  |
//orders table
| ord_id | pe_id | ord_title   |
|   1    |   1   | First order |
|   2    |   2   | Order two   |
|   3    |   2   | Third order |
//items table
| item_id | ord_id | pe_id | title  |
|   1     |   1    |   1   | Apple  |
|   2     |   1    |   1   | Pear   |
|   3     |   2    |   2   | Apple  |
|   4     |   3    |   2   | Orange |
|   5     |   3    |   2   | Coke   |
|   6     |   3    |   2   | Cake   |

我需要有一个列出所有人的查询，计算订单数量和 总计 项目数，例如:

| pe_name | num_orders | num_items |
| Foo  |    1       |   2       |
| Bar  |    2       |   4       |

但我不能让它工作!
我试过

SELECT
    people.pe_name,
    COUNT(orders.ord_id) AS num_orders,
    COUNT(items.item_id) AS num_items
FROM
    people
    INNER JOIN orders ON (orders.pe_id = people.pe_id)
    INNER JOIN items ON items.pe_id = people.pe_id
GROUP BY
    people.pe_id;

但这会返回 num_*值不正确:

| name | num_orders | num_items |
| Foo  |    2       |   2       |
| Bar  |    8       |   8       |

我注意到，如果我尝试一次加入一张 table ，它会起作用:

SELECT
    people.pe_name,
    COUNT(orders.ord_id) AS num_orders
FROM
    people
    INNER JOIN orders ON (orders.pe_id = people.pe_id)
GROUP BY
    people.pe_id;

//give me:
| pe_name | num_orders |
| Foo     |          1 |
| Bar     |          2 |

//and:
SELECT
    people.pe_name,
    COUNT(items.item_id) AS num_items
FROM
    people
    INNER JOIN items ON (items.pe_id = people.pe_id)
GROUP BY
    people.pe_id;
//output:
| pe_name | num_items |
| Foo     |         2 |
| Bar     |         4 |

如何将这两个查询合二为一？

Answer 1

将项目与订单加入比与人加入更有意义!

SELECT
    people.pe_name,
    COUNT(distinct orders.ord_id) AS num_orders,
    COUNT(items.item_id) AS num_items
FROM
    people
    INNER JOIN orders ON orders.pe_id = people.pe_id
         INNER JOIN items ON items.ord_id = orders.ord_id
GROUP BY
    people.pe_id;

与人们一起加入这些元素会引起大量的金币。
例如，订单 3 中的蛋糕项目将通过人与人之间的连接与订单 2 相关联，您不希望发生这种情况!!

所以 :

1- 您需要很好地了解您的架构。项目链接到订单，而不是人。

2-您需要为一个人计算不同的订单，否则您将计算与订单一样多的项目。