mysql语句

1.创建索引（unique 唯一）

create index index_name on emp(ename);

create unique index index_name on emp(ename);

2.添加索引
alter table emp add index index_name(ename);

alter table emp add unique (ename);

3.添加主键

alter table emp add primary key (eno)

4.删除索引

drop index index_name on emp;

5.删除表

drop table emp;

6.删除视图

drop view v_eno;

7.添加视图
alter table emp drop view v_eno;



8.查询sql语句
select * from emp where eno in(select ename from emp 
group by ename having count(name)>1) and 
people not in (select min(people)from people group by
name having count(name)>1)



9.查询前三名

select top 3 * from emp order by grade