SQL：按未连接的表分组

Question

I would like to display a list of movie titles that have been rented more than once at a DVD store. The 'rental' and 'film' tables are not connected though as you can see here: http://www.postgresqltutorial.com/wp-content/uploads/2013/05/PostgreSQL-Sample-Database.png

Here is what I have so far, which gives the error 'rentCount does not exist'

SELECT title, COUNT(rental_id) AS rentCount
FROM film f, rental r
WHERE rentCount >= 1
GROUP BY title
ORDER BY rentCount

Answer 1

(Unsure what kind of SQL you're using, however in SQL or MySQL, this should work)

It seems that the link from your film to rental table is through your inventory table. Join film to inventory using film_id , then join inventory to rental using inventory_id .

SELECT title, COUNT(rental_id) AS rentCount
FROM film f
INNER JOIN inventory i ON i.film_id = f.film_id
INNER JOIN rental r ON i.inventory_id = r.inventory_id
HAVING rentCount >= 1
GROUP BY title
ORDER BY rentCount

EDIT: Thanks to Nick's point out in the comments, I've changed your WHERE to be a HAVING , the reason being that you're using an aggregate function column ( rentCount ).

Answer 2

You need a HAVING clause:

SELECT f.title, COUNT(*) AS rentCount
FROM film f
INNER JOIN inventory i 
ON i.film_id = f.film_id
INNER JOIN rental r 
ON i.inventory_id = r.inventory_id
GROUP BY f.title
HAVING COUNT(*)  > 1
ORDER BY f.title

Notice I did not reference the alias "rentCount" in the HAVING.

Answer 3

SQL will not allow to use the use the column alias in the filter condition of the same query.if you wanted to use the column name, you need to use as a subquery like below.

SELECT *
 FROM
   (SELECT title, COUNT(r.rental_id) AS rentCount
  FROM film f
      JOIN Inventory I on f.film_id=I.film_id
    JOIN rental r on l.Inventory_id= r.Inventory_id
  GROUP BY title)t
  WHERE rentCount >= 1
  ORDER BY rentCount

You can also use a HAVING clause if you are using an agrgregate function and wanted to make the filter within the query.

  SELECT *
 FROM
   (SELECT title, COUNT(r.rental_id) AS rentCount
  FROM film f
      JOIN Inventory I on f.film_id=I.film_id
    JOIN rental r on l.inventory_id= r.inventory_id
  GROUP BY title
   Having Count(rental_id)>=1)t
  ORDER BY rentCount

Always keep in mind that Never use commas to link or make relation between the tables.Write appropriate joins and link them.In your case if you are using INENER JOIN there is no need of the filer condition 'Where rentCount>=1' since the condition will be always return a true value. I mean the below query will give you the same result like the others.

 SELECT title, COUNT(r.rental_id) AS rentCount
  FROM film f
      JOIN Inventory I on f.film_id=I.film_id
      JOIN rental r on l.inventory_id= r.inventory_id
  GROUP BY title