mysql常用sql语法

一、创建主键的三种方式

1、

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

2、

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

3、

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

4、联合主键

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

5、删除主键

ALTER TABLE USER DROP PRIMARY KEY;