SQL 如何找到哪个客户租了最多的电影？

Question

I'm struggling with a question that said Which customer has rented the most films?

I am doing this using the Seikila sample database in MySQL. I have something that joins my tables together and attempts to give me a count but I know its wrong just looking at the actual data in the rental table. my code is as below

SELECT r.rental_id, cust.customer_id, count(*) as Total_Rentals
FROM rental as r
INNER JOIN customer AS cust on r.customer_id = cust.customer_id
GROUP BY cust.customer_id;

but it tells me for example customer 1 has rented 32 movies, which I know is wrong. what am I doing wrong? since I was asked for clarification, the database I am using is: https://dev.mysql.com/doc/sakila/en/

And I am trying to find which customer has rented the most films, I am not entirely sure what my script is actually returning.

Answer 1

Remove the column rental_id from the select list and sort the result by count(*) descending to return the top 1 row:

SELECT cust.customer_id, cust.name, count(*) as Total_Rentals
FROM rental as r
INNER JOIN customer AS cust on r.customer_id = cust.customer_id
GROUP BY cust.customer_id, cust.name
ORDER BY Total_Rentals DESC LIMIT 1

But if you only need the customer's id then there is no need for a join:

SELECT customer_id, count(*) as Total_Rentals
FROM rental
GROUP BY customer_id
ORDER BY Total_Rentals DESC LIMIT 1

Answer 2

You need to join customer and rental , group by customer id (without rental id) and count it:

SELECT cust.customer_id, count(*) as Total_Rentals
FROM rental as r
INNER JOIN customer AS cust on r.customer_id = cust.customer_id
GROUP BY cust.customer_id;

So this code should work. If it doesn't work, that probably means that you have duplicates or other nonconventional issues.