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);