联合查询

基本语法：
select 语句1
union [union 选项]
select 语句2……

union选项:()
all:保留所有,不管重复
distinct:去重,默认

--同一张表
--联合查询多张表查询 去重
select * from my_class 
union
select * from my_class;
--不去重
select * from my_class 
union all --不去重
select * from my_class;

--不相同的表查询的内容也不相同
select id,c_name,room from my_class 
union all --不去重
select name,number,id from my_student;

下面我们做一个小练习

--男生升序,女生降序
(select * from my_student where sex='男' order by age asc limit 99999999)
union
(select * from my_student where sex='女' order by age desc limit 99999999)

按位置分类

from子查询

where子查询

exists 子查询

按结果分类

标量子查询

--一行一列
select * from my_student where c_id =(select id from my_class where c_name="python1809");

l列子查询

select * from my_student where c_id 
in--多个数据源
(select id from my_class);

行子查询

select * from my_student where age =(elect max(age)  from my_student)
ang
height =(select max(age) ,max(heiht) from my_student);


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

select * from my_student  order by age besc,height desc limit 1;

表子查询

select * from my_student where
exists (select * from my_class where id=1);



select * from my_student where
exists (select * from my_class where id=2);