使用两个外键作为主键 - MySQL [英] Using two foreign keys as a primary key - MySQL

问题描述

我是新的MySQL（不得不学习它uni）。
我必须为一个赋值创建一个数据库和web接口。

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.

在其中一个表上有两列，两列都是外键，我需要使用它们作为主键。

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.

这是迄今为止的代码：

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)
);

所有表语法都与提供的数据字典一致。

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

当我在mysql命令行中运行这个时，我在上面粗体突出的部分（表AuthorRole）上得到一个错误，说它不能添加外键约束。

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".

我试过调试它，它似乎是：

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

FOREIGN KEY(roleid) REFERENCES Role(id)

导致问题的外键我删除它，一切都很好，如果我离开它，并删除其他外键，它给一个错误。）

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).

如果有人可以请解释我在哪里我会非常感谢。

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

我试过谷歌搜索这个，但没有找到任何东西（可能是因为我使用错误的关键字）。

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

感谢

干杯
Corey

Cheers Corey

推荐答案

首先创建表Role，然后创建表AuthorRole，并且它会正常

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)
);

创建主键时最好使用 id INT NULL AUTO_INCREMENT

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