了解InnoDB外键

Question

I am following one PHP & MySQL tutorial. At this point, I have to create a database that should look like this:

I created the databases by hand, but I do not understand the point of the roll table. For instance, let's say I want to add a movie: how am I going to do it, since I am not allowed? Example error:

#1452 - Cannot add or update a child row: a foreign key constraint fails (`test`.`movie`, CONSTRAINT `movie_ibfk_1` FOREIGN KEY (`movieCode`) REFERENCES `roll` (`movieCode`) ON DELETE CASCADE) 

I would personally do something like:

Table artist: artistId , firstname, lastname, nationality, dateOfBirth, otherInfo Table movie: movieCode, title, image, category, description, artistId

Being the one in bold the related foreign keys. However, I do not understand the concept of using the roll table there. Can someone clarify this for me, as I'd like to do it like the tutorial pretends to teach?

DB schema I have so far:

SET SQL_MODE="NO_AUTO_VALUE_ON_ZERO";

CREATE TABLE IF NOT EXISTS `artist` (
  `artistId` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `firstName` char(30) NOT NULL,
  `lastName` char(30) NOT NULL,
  `dateOfBirth` date NOT NULL,
  `nationality` char(30) NOT NULL,
  `otherInfo` text NOT NULL,
  PRIMARY KEY (`artistId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;


CREATE TABLE IF NOT EXISTS `movie` (
  `movieCode` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `title` char(30) NOT NULL,
  `image` varchar(50) NOT NULL,
  `category` char(50) NOT NULL,
  `movieDesc` text NOT NULL,
  PRIMARY KEY (`movieCode`),
  UNIQUE KEY `title` (`title`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;


CREATE TABLE IF NOT EXISTS `roll` (
  `movieCode` int(10) unsigned NOT NULL,
  `artistId` int(10) unsigned NOT NULL,
  PRIMARY KEY (`movieCode`,`artistId`),
  KEY `artistId` (`artistId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `user` (
  `userId` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `firstName` char(30) NOT NULL,
  `lastName` char(30) NOT NULL,
  `username` char(30) NOT NULL,
  `password` char(20) NOT NULL,
  `usertype` int(1) unsigned NOT NULL,
  PRIMARY KEY (`userId`),
  UNIQUE KEY `username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;



ALTER TABLE `movie`
  ADD CONSTRAINT `movie_ibfk_1` FOREIGN KEY (`movieCode`) REFERENCES `roll` (`movieCode`) ON DELETE CASCADE;

ALTER TABLE `roll`
  ADD CONSTRAINT `roll_ibfk_1` FOREIGN KEY (`artistId`) REFERENCES `artist` (`artistId`) ON DELETE CASCADE;

Answer 1

The roll table (which should probably be called a role table) is what's known as a many-to-many relationship.

It stores a link between a movie and an artist. Since a movie can have many artists, and an artist can appear in many movies, you need a separate table to store all those relationships. Each row in roll represents an artist appearing in a movie.

To avoid any constraint errors, you would need to insert a movie and an artist in the database first, and then insert a row into the roll table to define that artist appeared in that movie. So, the roll table would need two foreign key constraints; one on the artist table, which you have:

ALTER TABLE `roll`
  ADD CONSTRAINT `roll_ibfk_1` FOREIGN KEY (`artistId`) REFERENCES `artist` (`artistId`) ON DELETE CASCADE;

And another on the movie table:

ALTER TABLE `roll`
  ADD CONSTRAINT `roll_ibfk_2` FOREIGN KEY (`movieCode`) REFERENCES `movie` (`movieCode`) ON DELETE CASCADE;

With these constraints, you won't be able to define a role unless both that artist and movie exist.

Answer 2

Your foreign key on movieCode should be on the roll table. MovieCode is the primary key of the movie table.

Answer 3

In your given example you have to generate the foreign key constraints from the roll (role) table to the other tables (movie, artist), not the other way around.

The given error message

#1452 - Cannot add or update a child row: a foreign key constraint fails (`test`.`movie`, CONSTRAINT `movie_ibfk_1` FOREIGN KEY (`movieCode`) REFERENCES `roll` (`movieCode`) ON DELETE CASCADE)

implies that you have a foreign key constraint on movie which refers the roll (role) table.

This would have the semantic that only a movie can be entered for which a role (an entry in roll) exists.

But I guess the semantics should be that you can only add roles for existing movies and existings artists. Therefore the roll (role) table should have foreign keys on the other tables, then you could add artists and movies, and then enter entries in roll (role) for valid artist-movie pairs.