Question

This a follow up to the question I posed here. Using the same kind of query, how do I make sure I get 10 movies back if there are restriction like ‘genre.name = ‘Classic Movies’ and ‘star.name = Humphrey Bogart.’ To be clear, the query must return data for 10 movies regardless of what the filtering restrictions are.

SELECT movie.id, movie.title, star.name, star.name_url, dir.name, 
       dir.name_url, genre.name, genre.name_url 
FROM 
        (SELECT * FROM movie WHERE movie.id > 0 ORDER BY movie.id LIMIT 10) movie
        LEFT JOIN actor 
             ON (movie.id = actor.movie_id) 
        LEFT JOIN person AS star 
             ON (actor.person_id = star.id) 
        LEFT JOIN director 
             ON (movie.id = director.movie_id) 
        LEFT JOIN person AS dir 
             ON (director.person_id = dir.id) 
        LEFT JOIN genre_classification 
             ON (movie.id = genre_classification.movie_id) 
        LEFT JOIN genre 
             ON (genre_classification.genre_id = genre.id)

Answer 1

The issue is you’re selecting a list of 10 movies from the database with this sub-select:

(SELECT * FROM movie WHERE movie.id > 0 ORDER BY movie.id LIMIT 10) movie

If you’re trying to filter down that list more using a WHERE clause outside that sub-select, you could potentially get less than those 10 rows, as not all 10 could match your criteria. You could add that new criteria to the sub-select like so:

(SELECT * FROM movie m INNER JOIN genre_classification g on g.movie_id = m.id and g.name = 'Classic Movies' WHERE m.id > 0 ORDER BY m.id LIMIT 10) movie

This would do the same thing, but only give you (up to) 10 movies who had a matching genre of “Classic Movies”.

This will, of course, still return more than 10 rows since you’re JOINing in actors, directors, other genres, etc. You should, however, have up to ten distinct movie IDs in the result set.

Full Query:

SELECT movie.id, movie.title, star.name, star.name_url, dir.name, 
       dir.name_url, genre.name, genre.name_url 
FROM 
        (SELECT * FROM movie m INNER JOIN genre_classification g on g.movie_id = m.id and g.name = 'Classic Movies' WHERE m.id > 0 ORDER BY m.id LIMIT 10) movie
        LEFT JOIN actor 
             ON (movie.id = actor.movie_id) 
        LEFT JOIN person AS star 
             ON (actor.person_id = star.id) 
        LEFT JOIN director 
             ON (movie.id = director.movie_id) 
        LEFT JOIN person AS dir 
             ON (director.person_id = dir.id) 
        LEFT JOIN genre_classification 
             ON (movie.id = genre_classification.movie_id) 
        LEFT JOIN genre 
             ON (genre_classification.genre_id = genre.id)