Question

I need to get data from 3 different tables. I know the basics about JOINs but when it comes to more complicated queries like getting data from 3 or more tables using JOIN I get a little confused and I just start playing with the queries writing what make sense to me, like the next one:

SELECT movies.imdbID,
  movies.title,
  movies.year,
  movie_ratings.votes,
  movie_ratings.total_value,
  movie_ratings_external.votes,
  movie_ratings_external.total_value
FROM movies, movie_ratings_external
LEFT JOIN movie_ratings ON movie_ratings.imdbID = movie_ratings_external.imdbID
WHERE movies.imdbID = movie_ratings_external.imdbID
ORDER BY movie_ratings.votes DESC, movie_ratings_external.votes DESC
LIMIT 30

This query works. I get the selected fields from the correct tables and ordered the right way, but I think I’m mixing up things (like regular querying two tables and JOINing between two tables) and I’m sure theres a better/more efficient way to accomplish the same purpouse.

Any DB geek available?

Answer 1

Modified your query just a tad bit. Everything else was pretty much spot on.

I am not exactly sure what you call a “regular query” but Joins are pretty much the very basic part of a regular query.

In your old query, this part FROM movies, movie_ratings_external was basically a CROSS JOIN. Replaced it with a JOIN (i used a LEFT JOIN assuming that you always wanted to return all the movies but basic intention was that the CROSS JOIN with the filtering in the WHERE is a LOT more inefficient because you are manipulating more ROWS – especially unnecessary ones)

Think of it this way. Every Join statement in your SELECT returns a set of row which are then JOINED with the next table in the JOIN. The idea should be to try an filter rows progressively with each join as far as possible so that you arent getting unnecessary tuples. Thats where the ON statements come in.

Removed

SELECT 
  movies.imdbID,
  movies.title,
  movies.year,
  movie_ratings.votes,
  movie_ratings.total_value,
  movie_ratings_external.votes,
  movie_ratings_external.total_value
FROM movies 
LEFT OUTER JOIN movie_ratings_external
     ON movies.imdbID = movie_ratings_external.imdbID
LEFT JOIN movie_ratings 
     ON movie_ratings.imdbID = movie_ratings_external.imdbID
ORDER BY movie_ratings.votes DESC, movie_ratings_external.votes DESC
LIMIT 30

Hope that helps!!