[英]SQL how to count total rows for each customer with group by?
我正在为一个问题而苦苦思索:“哪个城市的客户观看的电影最多?(根据租借的电影数量)”。
我正在使用MySQL中的Seikila示例数据库来执行此操作。 城市实体与租金和客户实体没有关系,但是有一个地址实体包含address_id,而id取决于客户实体。 然后,地址实体具有一个名为city_id的属性,城市实体也已获得该属性。 并且customer_id属性链接了客户实体和租赁实体。
另外,我必须计算每个城市中每个客户租用的电影总数。
我只完成了连接部分(代码已更新):
select rental_id,cust.customer_id,city.city_id,city.city from rental as r
inner join customer as cust on r.customer_id=cust.customer_id inner join
address as a on cust.address_id=a.address_id inner join city as city on
city.city_id=a.city_id
我意识到我应该将city_id添加到表中,所以我的问题将是:
如何计算唯一的customer_id的行数,这样我就可以得出每个城市 (按组) 租借的电影的总数 ?
使用COUNT(*)
获取电影租借总数,然后按城市分组。
SELECT city.city, COUNT(*) AS total_rentals
FROM rental AS r
INNER JOIN customer AS cust on r.customer_id = cust.customer_id
INNER JOIN address AS a on cust.address_id = a.address_id
INNER JOIN city ON city.city_id = a.city_id
GROUP BY city.city_id
ORDER BY total_rentals DESC
LIMIT 1
如果您还想知道该城市的客户数量,可以将COUNT(DISTINCT cust.customer_id)
添加到SELECT
列表中。
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