Question

I have a MySQL database with two tables (simplified for this question):
movies table columns: id, title, duration, description, created_at, updated_at
rentals table columns: id, movie_id, status, created_at, updated_at

The rental status is either ‘CHECKED OUT’ or ‘RETURNED’, so a movie is available if it either has no associated rental, or all of its associated rentals have a status of ‘RETURNED’. How do I query available movies? Right now I’ve got this:

SELECT rentals.id,rentals.status,movies.* 
FROM `movies` 
LEFT OUTER JOIN rentals ON movies.id = rentals.movie_id 
        AND movies.kiosk_id = rentals.kiosk_id 
WHERE (`movies`.kiosk_id = 1 
        AND (rentals.id is null or rentals.status != 'CHECKED OUT'));

The problem is this query returns multiple records for a movie if it’s been checked out several times, so my ORM gives me duplicate movie objects. Is there some way to specify that I only want unique movie.id‘s? Should I have some different database schema? Or should I sort it out programatically (seems like it would be too slow)?

Answer 1

I’m not sure what you want with the rentals id, so I took it out.

SELECT
  rentals.status, movies.*
FROM movies
LEFT OUTER JOIN (
  SELECT
    movie_id,
    kiosk_id,
    -- Because 'CHECKED OUT' comes alphabetically before 'RETURNED',
    -- this will have 'CHECKED OUT' if there's at least one rental checked out
    MIN(status) AS status
  FROM rentals
  GROUP BY movie_id, kiosk_id) rentals
ON movies.id = rentals.movie_id
AND movies.kiosk_id = rentals.kiosk_id
WHERE movies.kiosk_id = 1
AND (rentals.id is null or rentals.status != 'CHECKED OUT');