select * from user order by rand() limit 100 ##《随机读》读取100条数据
select * from user_info where uid in(select id from user where age<=18) ##《子查询》嵌套子查询作为筛选条件
select u.name,u.sex,i.school,i.score from user as u left join user_info as i on u.id=i.uid where u.name="帅哥陈" ##《联合查询-左连接》
