MySQL子查询
语法
语法：select 语句 where 条件 (select … from 表)
- 外面的查询称为父查询，括号中的查询称为子查询
- 子查询为父查询提供查询条件
基本使用子查询
1、查找笔试80分的学生
mysql> select * from stuinfo where stuno=(select stuno from stumarks where writtenexam=80);
+--------+---------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+---------+--------+--------+---------+------------+
| s25303 | 李斯文 | 女 | 22 | 2 | 北京 |
+--------+---------+--------+--------+---------+------------+
2、查找笔试最高分的学生
# 方法一：
mysql> select * from stuinfo where stuno=(select stuno from stumarks order by writtenexam desc limit 1);
+--------+---------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+---------+--------+--------+---------+------------+
| s25303 | 李斯文 | 女 | 22 | 2 | 北京 |
+--------+---------+--------+--------+---------+------------+
1 row in set (0.00 sec)
# 方法二：
mysql> select * from stuinfo where stuno=(select stuno from stumarks where writtenexam=(select max(writtenexam) from stumarks));
+--------+---------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+---------+--------+--------+---------+------------+
| s25303 | 李斯文 | 女 | 22 | 2 | 北京 |
+--------+---------+--------+--------+---------+------------+
1 row in set (0.00 sec)
脚下留心：上面的例题，子查询只能返回一个值。如果子查询返回多个值就不能用“=”了,需要用 in
in|not in子查询
用于子查询的返回结果多个值。
1、查找笔试成绩及格的同学
mysql> select * from stuinfo where stuno in (select stuno from stumarks where writtenexam>=60);
+--------+----------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+----------+--------+--------+---------+------------+
| s25301 | 张秋丽 | 男 | 18 | 1 | 北京 |
| s25303 | 李斯文 | 女 | 22 | 2 | 北京 |
| s25304 | 欧阳俊雄 | 男 | 28 | 4 | 天津 |
+--------+----------+--------+--------+---------+------------+
3 rows in set (0.00 sec)
2、查询不及格的同学
mysql> select * from stuinfo where stuno in (select stuno from stumarks where writtenexam<=60);
+--------+----------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+----------+--------+--------+---------+------------+
| s25302 | 李文才 | 男 | 31 | 3 | 上海 |
| s25318 | 争青小子 | 男 | 26 | 6 | 天津 |
+--------+----------+--------+--------+---------+------------+
3、查询没有通过的同学（不及格，缺考）
mysql> select * from stuinfo where stuno not in (select stuno from stumarks where writtenexam>=60);
+--------+----------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+----------+--------+--------+---------+------------+
| s25302 | 李文才 | 男 | 31 | 3 | 上海 |
| s25305 | 诸葛丽丽 | 女 | 23 | 7 | 河南 |
| s25318 | 争青小子 | 男 | 26 | 6 | 天津 |
| s25319 | 梅超风 | 女 | 23 | 5 | 河北 |
+--------+----------+--------+--------+---------+------------+
4 rows in set (0.00 sec)
exists和not exists子查询
1、 如果有人笔试超过80分就显示所有的学生
mysql> select * from stuinfo where exists (select * from stumarks where writtenexam>=80);
+--------+----------+--------+--------+---------+------------+
| stuNo | stuName | stuSex | stuAge | stuSeat | stuAddress |
+--------+----------+--------+--------+---------+------------+
| s25301 | 张秋丽 | 男 | 18 | 1 | 北京 |
| s25302 | 李文才 | 男 | 31 | 3 | 上海 |
| s25303 | 李斯文 | 女 | 22 | 2 | 北京 |
| s25304 | 欧阳俊雄 | 男 | 28 | 4 | 天津 |
| s25305 | 诸葛丽丽 | 女 | 23 | 7 | 河南 |
| s25318 | 争青小子 | 男 | 26 | 6 | 天津 |
| s25319 | 梅超风 | 女 | 23 | 5 | 河北 |
+--------+----------+--------+--------+---------+------------+
2、 如果没有人超过80分就显示所有的学生
mysql> select * from stuinfo where not exists (select * from stumarks where writtenexam>=80);
Empty set (0.02 sec)
子查询分类
1、标量子查询：子查询返回的结果就一个
2、列子查询：子查询返回的结果是一个列表
3、行子查询：子查询返回的结果是一行
例题：查询成绩最高的男生和女生
mysql> select stuname,stusex,ch from stu where (stusex,ch) in (select stusex,max(ch) from stu group by stusex);
+----------+--------+------+
| stuname | stusex | ch |
+----------+--------+------+
| 争青小子 | 男 | 86 |
| Tabm | 女 | 88 |
+----------+--------+------+
4、表子查询：子查询返回的结果当成一个表
例题：查询成绩最高的男生和女生
mysql> select stuname,stusex,ch from (select * from stu order by ch desc) as t group by stusex;
+----------+--------+------+
| stuname | stusex | ch |
+----------+--------+------+
| Tabm | 女 | 88 |
| 争青小子 | 男 | 86 |
+----------+--------+------+
脚下留心：from后面是一个表，如果子查询的结果当成表来看，必须将子查询的结果取别名。