MySQL: M:N Table Linking Two Tables

Question

I'm getting ready to populate a couple of tables in a movie database I have created. The Movies table has already been created. It looks like this:

 movieid INT NOT NULL PRIMARY KEY
 title VARCHAR(50)
 link  VARCHAR(100)
 release_date VARCHAR(10)

It should perhaps be noted that I'm using MySQL-python and Scrapy to populate this data in the MySQL database, but perhaps that doesn't matter.

I have created three additional tables which have not yet been populated: Producers , Actors and Directors . Each of these looks like the other in that there exists id , name and movie title . So Actors looks like:

 actorid INT NOT NULL PRIMARY KEY
 actor   VARCHAR(40)
 title   VARCHAR(50)    <<<<------ This is the movie title from the Movies tables

Now, I would like to create a table called Movie To Actor which holds the movieid from the Movie table and the actorid from the Actor table and not have the title column existing in the Actors table.

I figure there are two ways to do this:

First Way:

Get all of the data into the Actors table with title acting as a foreign key and to THEN create the Movie To Actor table, adding the appropriate ids where movies.title is equal to Actors.title and to then drop the title column from Actors .

Second way:

Do this as I am populating the data. First create the Movie to Actor table, set up key constraints and set this to auto update as the data is added to Actors table. I would prefer this as it makes things significantly easier and I think is stylistically better. Was hoping someone could instruct me on how I would go about doing this. I think I can figure out how to extrapolate instructions in MySQL into MySQL-python , but would need help on how to do the former.

Answer 1

A couple points first off:

• Unless you're manually populating the actorid and movieid fields in the actors and movies tables you should have an auto_increment on those columns.
• If you're using movies.title to link to anything else, you should add a unique index to that column in the movies table.
• You should ultimately drop the title column from the actors table. Otherwise you'll either have multiple rows for the same actor name (redundant) or only one movie per actor (unrealistic).

For the first method you mentioned, you actually can first create the movie_actor table and then populate the existing data:

INSERT INTO movie_actor (movieid, actorid) SELECT movieid, actorid FROM movies INNER JOIN actors ON actors.title=movies.title;

For the second, you can use a trigger:

delimiter //
CREATE TRIGGER add_actor
AFTER INSERT ON actors
FOR EACH ROW BEGIN
SET @movieid = (SELECT movieid FROM movies WHERE title=NEW.title);
IF @movieid IS NOT NULL THEN
    INSERT INTO movie_actor (movieid, actorid) VALUES (@movieid, NEW.actorid);
END IF;
END;
//
delimiter ;

I'd suggest option 1 followed by having whatever form you're using to populate these tables to populate the movie_actor table independently (eg after adding an actor, you can set whatever movies he/she was in, merely adding a mapping to the movie_actor table). Then if you want to get a list of movies for an actor you can just

SELECT title FROM movie_actor
INNER JOIN movies ON movies.movieid=movie_actor.movieid
WHERE actorid=????