查找记录 Select  

select username,id from users;

Group by 进行分组

select sex from users group by sex;

分组条件

select age,sex from users  group by sex having age>21;

查询结果进行排序

Desc 降序

select * from users order by age,id desc;

限制查询结果返回的数量

select * from users limit 2,2;

返回第3、4条记录

Insert 配合select语句使用

insert test1(username) select username from users where age>-=21;