使用两个外键作为主键 - MySQL

Question

I am quite new to MySQL (have had to learn it for uni). I have to create a database and web interface for an assignment.

On one of the tables I have two columns, both of which are foreign keys, and i need to use them both as the primary key.

This is the code so far:

drop database if exists testJoke;
create database testJoke;
use testJoke;

CREATE TABLE Author
(
  id           int(11)   NOT NULL ,
  name         varchar(255) NULL ,
  cust_email   varchar(255) NULL,
  password char(32) null,

  PRIMARY KEY (id)


);


**CREATE TABLE AuthorRole
(
  authorid  int(11) NOT NULL ,
  roleid varchar(255) NOT NULL,
  PRIMARY KEY (authorid, roleid),
  FOREIGN KEY(authorid) REFERENCES Author(id),
  FOREIGN KEY(roleid) REFERENCES Role(id)

);**



CREATE TABLE Category
(
  id  int(11)      NOT NULL ,
  name varchar(255) NULL,
  PRIMARY KEY (id)
);


CREATE TABLE Joke
(
  id    int(11)      NOT NULL ,
  joketext   text    NULL ,
  jokedate    date   NOT NULL ,
  authorid int(11)   NULL,
  PRIMARY KEY (id),
  FOREIGN KEY(authorid) REFERENCES Author(id)

);


CREATE TABLE JokeCategory
(
  jokeid    int(11)      NOT NULL ,
  categoryid    int(11)  NOT NULL ,
  PRIMARY KEY (jokeid, categoryid),
  FOREIGN KEY(jokeid) REFERENCES Joke(id),
  FOREIGN KEY(categoryid) REFERENCES Category(id)**


);

CREATE TABLE Role
(
  id    varchar(255)      NOT NULL ,
  description  varchar(255)  NULL ,
  PRIMARY KEY (id)
);

All of the table syntax is in line with a data dictionary provided.

When i run this in the mysql command line, i get an error on the section highlighted in bold above (table "AuthorRole"), saying that it "cannot add foreign key constraint".

I have had a try at debugging it, and it seems to be the:

FOREIGN KEY(roleid) REFERENCES Role(id)

Foreign key that is causing the problem (if i remove it, all works well, and if i leave it in and remove the other foreign key, it gives an error).

If someone could please explain where i am going wrong, i would be very grateful.

I have tried googling this, but was unable to find anything (probably because i was using the wrong keywords).

Thanks

Cheers Corey

Answer 1

At first create the table "Role", then the table "AuthorRole" and it'll be ok

CREATE TABLE Role
(
  id    varchar(255)      NOT NULL ,
  description  varchar(255)  NULL ,
  PRIMARY KEY (id)
);

CREATE TABLE AuthorRole
(
  authorid  int(11) NOT NULL ,
  roleid varchar(255) NOT NULL,
  PRIMARY KEY (authorid, roleid),
  FOREIGN KEY(authorid) REFERENCES Author(id),
  FOREIGN KEY(roleid) REFERENCES Role(id)
);

And when creating primary keys it's better to use id INT(11) NOT NULL AUTO_INCREMENT

Answer 2

It is simply because you are referring to a primary key of the table which is not created at the time you refer to it, try the following sql script:

drop database if exists testJoke;
create database testJoke;
use testJoke;

CREATE TABLE Author
(
   id           int(11)   NOT NULL ,
   name         varchar(255) NULL ,
   cust_email   varchar(255) NULL,
   password char(32) null,

   PRIMARY KEY (id)


);

CREATE TABLE Role
(
   id    varchar(255)      NOT NULL ,
   description  varchar(255)  NULL ,
   PRIMARY KEY (id)
);


CREATE TABLE AuthorRole
(
   authorid  int(11) NOT NULL ,
   roleid varchar(255) NOT NULL,
   PRIMARY KEY (authorid, roleid),
   FOREIGN KEY(authorid) REFERENCES Author(id),
   FOREIGN KEY(roleid) REFERENCES Role(id)

);



CREATE TABLE Category
(
  id  int(11)      NOT NULL ,
  name varchar(255) NULL,
  PRIMARY KEY (id)
);


CREATE TABLE Joke
(
  id    int(11)      NOT NULL ,
  joketext   text    NULL ,
  jokedate    date   NOT NULL ,
  authorid int(11)   NULL,
  PRIMARY KEY (id),
  FOREIGN KEY(authorid) REFERENCES Author(id)

);


CREATE TABLE JokeCategory
(
  jokeid    int(11)      NOT NULL ,
  categoryid    int(11)  NOT NULL ,
  PRIMARY KEY (jokeid, categoryid),
  FOREIGN KEY(jokeid) REFERENCES Joke(id),
  FOREIGN KEY(categoryid) REFERENCES Category(id)


);

Create table Role first and then AuthorRole.

Answer 3

try changeing

  roleid varchar(255) NOT NULL,

to

  roleid int(11) NOT NULL,

as the field and your key field are of different types