在一个查询中聚合来自两个不同级别的信息

Question

我想知道是否可以仅使用一个查询来聚合两个不同级别的信息？ 例如，我有一张表，想要获得购买某个商品的唯一客户数量以及每个 customer_id 购买的某个 item_id 的数量除以客户总数。

Table
customer_id   item_id    bought_date
   abc           12        2017-01-01
   def           23        2017-01-08
   abc           12        2017-01-02
   abc           13        2017-01-02
   ghi           23        2017-01-02

我想要输出

item_id   customer_id   item_count_per_customer customers_probability_per_item total_customers
 12         abc               2                      1        3
 13         abc               1                      1        3
 23         def               1                      2        3
 23         ghi               1                      2.

我可以按如下方式获取单个列 item_count_per_customer：

select item_id, customer_id, count(1) as item_count_per_customer
from table
group by item_id, customer_id

我还可以获取单个列customers_count_per_item，如下所示： 选择 item_id，count(distinct customer_id) 作为 customers_count_per_item 从表 按 item_id 分组

我还需要如下的唯一客户总数： 从表中选择 count(distinct customer_id) 作为 total_customers

所以我需要将所有这些信息集中在一起。做到这一点的唯一方法是将这 3 个查询（可能作为子查询）结合起来，还是有更有效的方法来做到这一点？

Answer 1

窗口函数

select      item_id
           ,customer_id
           ,count(*)                                                as item_count_per_customer 
           ,count(distinct customer_id) over (partition by item_id) as customers_count_per_item
           ,count(distinct customer_id) over()                      as total_customers

from        mytable

group by    item_id
           ,customer_id
;

---

+---------+-------------+-------------------------+--------------------------+-----------------+
| item_id | customer_id | item_count_per_customer | customers_count_per_item | total_customers |
+---------+-------------+-------------------------+--------------------------+-----------------+
| 23      | ghi         | 1                       | 2                        | 3               |
+---------+-------------+-------------------------+--------------------------+-----------------+
| 23      | def         | 1                       | 2                        | 3               |
+---------+-------------+-------------------------+--------------------------+-----------------+
| 13      | abc         | 1                       | 1                        | 3               |
+---------+-------------+-------------------------+--------------------------+-----------------+
| 12      | abc         | 2                       | 1                        | 3               |
+---------+-------------+-------------------------+--------------------------+-----------------+