究竟什么是外键? [英] What exactly is a foreign key?

问题描述

好的.所以我知道数据库中的主键是什么.如果您在数据库中有一个表，则主键是表中每一行唯一的单个值.例如:

Ok. So I know what a primary key in DB is. If you have a table in a database, a primary key is a single value that is unique to each row in your table. For example:

id   | name    | whatever
-------------------------
1      Alice     ....
2      Bob       ....
45     Eve       ....
988    ....      ....

所以我需要一个好的、简单的例子来解释外键到底是什么.因为我就是不明白:)

So I need a good, simple example to explain what exactly a foreign key is. Because I just don't get it :)

好的，这很简单，我想我把问题复杂化了.

OK it's pretty easy, I guess I was over-complicating the problem.

最后一个问题，外键的唯一限制是它们是我所指的表中的有效主键值吗?

So one final question, the only restriction on foreign keys is that it they are a valid primary key value in the table I am referring to?

推荐答案

外键是指向另一个表的主键的字段.

A foreign key is a field that points to a primary key of another table.

例子:

Table Name - Users

UserID    UserName    UserRoleID
1         JohnD       1
2         CourtneyC   1
3         Benjamin    2

Table Name - UserRoles

UserRoleID    Desc
1             Admin
2             Moderator

您可以看到 Users.UserRoleID 是一个外键，它指向主键 UserRoles.UserRoleID

You can see that Users.UserRoleID is a foreign key which points to the primary key UserRoles.UserRoleID

外键的使用使得在其他表上建立关系变得简单，允许您以一种很好的方式将多个表的数据链接在一起:

The use of foreign keys makes setting up relationships on other tables simple, allowing you to link together the data of multiple tables in a nice way:

例子:

SELECT
    a.UserID, 
    a.UserName, 
    b.Desc as [UserRole]
FROM 
    Users a INNER JOIN 
        UserRoles b ON a.UserRoleID = b.UserRoleID

然后输出将是:

UserID    UserName    User Role
1         JohnD       Admin
2         CourneyC    Admin
3         Benjamin    Moderator

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