在三个表上使用count()进行内部联接 [英] Inner join with count() on three tables

问题描述

简单快速的问题，我有那些表:

Simple and fast question, i have those tables:

//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   |

我需要一个查询列出所有人员，计算订单数和总数项数，如下所示:

I need to have a query listing all the people, counting the number of orders and the total number of items, like that:

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

但是我无法使其正常工作！ 我尝试过

But i can not make it work! I tried

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;

但是这会返回错误的num_*值:

But this returns the num_* values incorrect:

| 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桌子，它将起作用:

I noticed that if i try to join one table at time, it works:

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而为一?

How to combine those two queries in one?

推荐答案

将商品与订单一起加入比与人一起来更有意义！

It makes more sense to join the item with the orders than with the people !

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;

与人们一起加入物品会招来很多麻烦. 例如，订单3中的蛋糕将通过人与人之间的联接而与订单2链接，而您不希望这种情况发生！

Joining the items with the people provokes a lot of doublons. For example, the cake items in order 3 will be linked with the order 2 via the join between the people, and you don't want this to happen !!

所以:

1-您需要对架构有一个很好的了解.物料链接到订单，而不是人员.

1- You need a good understanding of your schema. Items are link to orders, and not to people.

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商品.

2- You need to count distinct orders for one person, else you will count as many items as orders.

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