一.联合查询：将多次查询(多条select语句)，在记录上拼接(字段不会增加)
1. 基本语法：多条select构成，每条select语句字段数保持一致(字段类型可以不一致)
1)：select 语句1 union[选项] select 语句2 union[选项]
2)：union[选项]：all(不管重复)保留所有、distinct去重(必须一条记录全部重复）默认
select * from my_class union distinct select name,sex,course from student;
3)：联合查询的意义
1.查询同一张表，但需求不同，男生降序，女生升序
在联合查询中使用order by查询两个查询语句都必须用(),搭配limit使用
(select * from student where sex='male' order by age asc limit 100) union (select * from student where sex='female' order by age desc limit 100);
2.查询多张表多张表的结构完全一样，保存的数据(结构)也一样
二.子查询：一个查询结果作为另一个查询的条件
1.子查询分类：按位置分类、结果分类
1)：按位置查询：子查询(select语句)在外部查询(select语句)中出现的位置。
1.from子查询：子查询在from之后
2.where子查询：子查询在where之后
3.exists子查询：子查询在exists里面
2)：按照结构子查询：根据子查询得到的结果进行分类
1.标量子查询：子查询得到的结果是一行一列,例如一致课程名称查询id
select * from student where id=(select id from student where course='Java');
+----+--------+------+------+--------+
| id | name | sex | age | course |
+----+--------+------+------+--------+
| 1 | steven | male | 20 | Java |
+----+--------+------+------+--------+
2.列子查询：子查询得到的结果是一列多行，需要使用in作为查询条件，类似的还有all、some、any
select * from student where id =some(select id from student);
使用all、some、any时必须在前面加=号
select * from student where id in(select id from student);
+----+--------+--------+------+--------+
| id | name | sex | age | course |
+----+--------+--------+------+--------+
| 1 | steven | male | 20 | Java |
| 2 | mary | female | 27 | PHP |
| 3 | candy | female | 24 | C语言 |
| 4 | mark | male | 22 | C++ |
| 5 | hary | female | 26 | JavaEE |
+----+--------+--------+------+--------+
3.行子查询：子查询得到的结果时多行一列(多行多列),需要构造行元素，它由多个字段构成
select * from student where id=(select max(id) from student) or age=(select max(age) from
student);
select * from student where (id,age)=(select max(id),max(age) from student);
4.表子查询：子查询得到的结果是多行多列(位置在from之后)，返回结果当作二维表来使用
select * from (select * from student order by height desc) as stu group by sex;
5.exists子查询：用来判断某些查询条件是否满足(跨表查询),exists实在where之后返回结果0和1
select * from student where exists (select * from my_class);