Composite primary keys are what you want where you want to create a many to many relationship with a fact table. For example, you might have a holiday rental package that includes a number of properties in it. On the other hand, the property could also be available as a part of a number of rental packages, either on its own or with other properties. In this scenario, you establish the relationship between the property and the rental package with a property/package fact table. The association between a property and a package will be unique, you will only ever join using property_id with the property table and/or package_id with the package table. Each relationship is unique and an auto_increment key is redundant as it won't feature in any other table. Hence defining the composite key is the answer.

Composite primary keys are what you want where you want to create a many to many relationship with a fact table. For example, you might have a holiday rental package that includes a number of properties in it. On the other hand, the property could also be available as a part of a number of rental packages, either on its own or with other properties. In this scenario, you establish the relationship between the property and the rental package with a property/package fact table. The association between a property and a package will be unique, you will only ever join using property_id with the property table and/or package_id with the package table. Each relationship is unique and an auto_increment key is redundant as it won't feature in any other table. Hence defining the composite key is the answer.

除了个人的设计偏好外，还有一些情况需要使用复合主键。表可以有两个或多个字段，提供唯一的组合，而且不一定是通过外键。

例如，美国每个州都有一组独特的国会选区。虽然许多州可能单独拥有一张CD-5，但在50个州中，永远不会有超过一张CD-5，反之亦然。因此，为Massachusetts CD-5创建一个自动编号字段是多余的。

如果数据库驱动一个动态网页，编写代码查询两个字段的组合可能比提取/重新提交一个自动编号的键要简单得多。

所以，虽然我没有回答最初的问题，但我很欣赏亚当的直接回答。

语法为CONSTRAINT constraint_name PRIMARY KEY(col1,col2,col3)，例如:

约束pk_PersonID主键(P_Id,LastName)

上面的例子将工作，如果你正在写它，而你正在创建表，例如:

CREATE TABLE person (
   P_Id int ,
   ............,
   ............,
   CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
);

要将此约束添加到现有表中，需要遵循以下语法

ALTER TABLE table_name ADD CONSTRAINT constraint_name PRIMARY KEY (P_Id,LastName)

我将使用复合(多列)键。

CREATE TABLE INFO (
    t1ID INT,
    t2ID INT,
    PRIMARY KEY (t1ID, t2ID)
) 

这样，t1ID和t2ID也可以作为指向各自表的外键。

我不会让“info”表的主键是来自其他表的两个值的组合。

其他人可以更好地阐述原因，但让一个实际上由两条信息组成的专栏感觉是错误的。如果出于某种原因想对来自第二个表的ID进行排序呢?如果想要计算两个表中的值出现的次数，该怎么办?

我总是把这两列分开。你可以在mysql中使用双列primay键…主键(id_a, id_b)…但我更喜欢使用两列唯一索引，并有一个自动递增的主键字段。