What are MySQL foreign keys?

Question

In an answer on Stack Overflow, I saw this code:

CREATE TABLE Favorites (
    user_id INT NOT NULL,
    movie_id INT NOT NULL,
    PRIMARY KEY (user_id, movie_id),
    FOREIGN KEY (user_id) REFERENCES Users(user_id),
    FOREIGN KEY (movie_id) REFERENCES Movies(movie_id)
);

I've never used the 'foreign key' relationship keyword before.

• What is it?
• Why do people use it?
• Does it provide any benefit apart from semantics?

Answer 1

A foreign key is a reference to a primary key in another table or the table itself. It is used for what is called referential integrity. Basically in your table provided, for a Record to be inserted into Favorites - you would have to supply a valid user_id from the Users table, and a valid movie_id from the Movies table. With Foreign keys enforces, I could not delete a record from Users or Movies. If I didn't have foreign keys, I could delete those records. Then if I did a SELECT ... JOIN on Favorites it would break.

See Wikipedia.