MySQL学习--唯一索引

唯一索引：就是创建索引时，限制索引的值必须唯一

 

1.在创建表时创建索引

create table emp(

ename varchar(20),

deptno int(10) primary key auto_increment,

unique index index_niu(deptno)

);

explain select * from emp where deptno=22;

 

2.已有表创建索引

create table emp(

ename varchar(20),

deptno int(10) primary key auto_increment

);

create unique index index_niu on emp(deptno);

explain select * from emp where deptno=22;

 

3.alter table创建索引

alter table emp add unique index index_niu(deptno);

 

4.删除索引

drop index index_niu on emp;