如何在 sql 中通过单个查询获取项目计数和总计数？

Question

我有一个 table 可以存储有关一家甜品店在所有地点销售的商品的信息。它出售糕点、华夫饼、冰淇淋、糖果等

我需要从 table 获取此信息。

Which item was sold the most per location along with total items sold in those locations.

下面是table

的结构

     Column     |            Type             | Modifiers 
----------------+-----------------------------+-----------
 id             | integer                     | 
 date           | timestamp without time zone | 
 item           | character varying(15)       | 
 location       | character varying(25)       |

现在我把它分解成两个单独的问题

每个地点哪个商品的销量最高
每个地点售出的商品总数。

这些是我的查询：

查询 #1：

select location, item, count(item) as count_ 
from sweet_shop 
group by location, item;

查询#2：

select location, count(item) as count_ 
from sweet_shop 
group by location;

如您所见，我进行了两个单独的查询，然后我需要将这两个信息映射到正确的位置并获得最终输出。我正在寻找如下所示的输出

  location     |  item             | item_count    total_count 
---------------+-------------------+-------------|----------
 BA2           | candies           |   400       |     550
 BA4           | pastries          |   320       |     610 
 BA3           | waffles           |   250       |     400 
 BA7           | ice creams        |   180       |     500

但我想在单个查询中获取此信息，而不是进行两个单独的查询。如何做到这一点？

注意：我使用的是 Postgres 9.2

Answer 1

我会按位置和项目进行汇总，使用 COUNT() 作为分析函数来生成每个位置的总计数。此外，使用 ROW_NUMBER 确定每个位置中表现最好的项目。

WITH cte AS (
    SELECT location, item, COUNT(*) AS item_count,
           SUM(COUNT(*)) OVER (PARTITION BY location) AS total_count,
           ROW_NUMBER() OVER (PARTITION BY location
                              ORDER BY COUNT(*) DESC) rn
    FROM sweet_shop
    GROUP BY location, item
)

SELECT location, item, item_count, total_count
FROM cte
WHERE rn = 1;

Answer 2

Postgres 有一个非常好的扩展 DISTINCT ON，它允许您在没有子查询的情况下执行此操作：

SELECT DISTINCT ON (location) location item,
       COUNT(*) AS item_count,
       SUM(COUNT(*)) OVER (PARTITION BY location) AS total_count
FROM sweet_shop
GROUP BY location, item
ORDER BY location, COUNT(*) DESC;

DISTINCT ON return 具有相同键的一组行的第一行，其中“第一”由 ORDER BY 子句定义。

如何在 sql 中通过单个查询获取项目计数和总计数？

How to get item count along with total count with a single query in sql?

sql

postgresql-9.2