MySQL PRIMARY KEY
1. 介绍¶
PRIMARY KEY 约束
- PRIMARY KEY 约束唯一标识数据库表中的每条记录。
- 主键必须包含唯一的值。
- 主键列不能包含 NULL 值。
- 每个表都应该有一个主键,并且每个表只能有一个主键。
1. 操作¶
1.1 实例¶
drop table info; create table info( id int not null primary key auto_increment, name varchar(20), age int not null, score int not null ); insert into info(name,age,score) values("张三",18,80); insert into info(name,age,score) values("张三",40,66); insert into info(name,age,score) values("李四",20,55); insert into info(name,age,score) values("王五",30,89); insert into info(name,age,score) values("赵六",40,77);
mysql> drop table info; Query OK, 0 rows affected (0.03 sec) mysql> create table info( -> id int not null primary key auto_increment, -> name varchar(20), -> age int not null, -> score int not null -> ); Query OK, 0 rows affected (0.05 sec) mysql> insert into info(name,age,score) values("张三",18,80); Query OK, 1 row affected (0.00 sec) mysql> insert into info(name,age,score) values("张三",40,66); Query OK, 1 row affected (0.00 sec) mysql> insert into info(name,age,score) values("李四",20,55); Query OK, 1 row affected (0.02 sec) mysql> insert into info(name,age,score) values("王五",30,89); Query OK, 1 row affected (0.02 sec) mysql> insert into info(name,age,score) values("赵六",40,77); Query OK, 1 row affected (0.00 sec) mysql> select * from info; +----+--------+-----+-------+ | id | name | age | score | +----+--------+-----+-------+ | 1 | 张三 | 18 | 80 | | 2 | 张三 | 40 | 66 | | 3 | 李四 | 20 | 55 | | 4 | 王五 | 30 | 89 | | 5 | 赵六 | 40 | 77 | +----+--------+-----+-------+ 5 rows in set (0.00 sec) mysql> desc info; +-------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | age | int(11) | NO | | NULL | | | score | int(11) | NO | | NULL | | +-------+-------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)