MySql-默认情况下主键是唯一的吗? [英] MySql - Is primary key unique by default?

问题描述

如果我在MySql中将一列定义为主键，则默认情况下它也是唯一键还是需要将其定义为唯一键(如果我希望它是唯一的)?

If I define a column as a primary key in MySql, is it also unique key by default or do I need to also define it as unique key (in case I want it to be unique)?

我看到了这个问题 b有什么区别/w主键和唯一键可以解释两者之间的区别，但不能完全回答我的问题. PK默认是英国还是我需要对其进行明确定义.

I saw this question What is the difference b/w Primary Key and Unique Key that explain the difference between the two but doesn't exactly answer my question. Does PK is UK by default or I need to explicitly define it.

推荐答案

Primary key is always unique in every SQL. You dont have to explicitly define it as UNIQUE.

附带说明::表中只能有一个主键，并且永远不允许空值.此外，表中只能有一个主键约束(因为创建主键的目的是唯一标识表中的行)，但是表中可以有多个唯一键约束.

On a side note: You can only have onePrimary key in a table and it never allows null values. Also you can have only one primary key constraint in the table(as the point of creating a primary key is to uniquely identify the row in your table) but you can more than one unique key constraint in your table.

示例:

以EmpID作为主键而EmpPhoneNo作为唯一键的员工详细信息表.

An employee details table having EmpID as Primary key and EmpPhoneNo as unique key.

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