第三件：mysql复杂语句

连接查询

　　-- inner join

　　-- left join

　　-- right join

　　-- full join

子查询

　　-- where子句后

　　　　标量子查询：返回结果 一列一行

　　　　　　select * from student where c_id = (select id from class where c_name = '六年级一班');

　　　　列子查询：返回结果 一列多行

　　　　　　select * from student where c_id in (select id from class);

　　　　　　select * from student where c_id =any (select id from class);

　　　　　　select * from student where c_id =some (select id from class);

　　　　　　select * from student where c_id =all (select id from class);　　-- 无结果

　　　　行子查询：返回结果 多列多行

　　　　　　构造行元素(age,height)

　　　　　　select * from student where (age,height) = (select max(age),max(height) from student);

　　-- from子句后

　　　　表子查询：返回结果当做二维表来使用

　　　　　　select * from (select * from student order by height desc) as a group by c_id;

　　-- exists子查询

　　　　select * from student exists(select * from class where id = 3);