mysql中创建主键的三种方式以及联合主键的创建方式

mysql中创建主键的三种方式

方式一

给uid设置主键

CREATE TABLE user(
   uid INT PRIMARY KEY,
   uname VARCHAR(10),
   address VARCHAR(20)
)

方式二

给uid设置主键

CREATE TABLE user(
   uid INT,
   uname VARCHAR(10),
   address VARCHAR(20),
   PRIMARY KEY(uid)
)

方式三

给uid设置主键

CREATE TABLE user(
 uid INT,
 uname VARCHAR(10),
 address VARCHAR(20)
)
ALTER TABLE user ADD PRIMARY KEY(uid);

联合主键创建方式

给uid和uname设置联合主键

CREATE TABLE user(
	uid INT,
	uname VARCHAR(10),
	address VARCHAR(20),
	PRIMARY KEY(uid,uname)
)

删除主键约束

ALTER TABLE USER DROP PRIMARY KEY;