mysql数据查询之子查询

子查询概念：

sub query，查询是在某个查询结果之上进行的，一条select语句内部包含了另外一条select语句。

(1)标量子查询 where 之后写 = ，确定某一个值

select * from student where c_id = (select id from class where grade = "PM3.1");

(2)列子查询    where 之后 写 in，是一列的所有值

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

(3)行子查询    两点确定一条直线 ，where之后放两个值，或者直接用max() 或 min()

• • select * from student where age = ? and height = ?;
• 然后确定最大年龄和最大身高。 
  
  
• • select max(age), max(height) from student；

(4)表子查询比较特殊，查询内容放在from之后，查询得到一个表，最好再有个别名

-- 表子查询

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

(5)exists子查询，放在where之后

select * from student where exists(select * from class);