【7】多表查询与子查询

1.查询所有学生的stu_name，cour_num，degree列：

其中stu_name字段来自于student表；cour_num和degree字段来自于score表；

mysql> select * from student;
+---------+----------+---------+---------------------+-------+
| stu_num | stu_name | stu_sex | stu_birth           | class |
+---------+----------+---------+---------------------+-------+
| 11215   | JiaWei   | F       | 1993-07-28 00:00:00 | 112   |
| 11328   | DingQi   | F       | 1994-08-15 00:00:00 | 113   |
| 11422   | Baker    | F       | 1999-09-22 00:00:00 | 114   |
| 11423   | Bob      | M       | 1998-04-25 00:00:00 | 114   |
| 11424   | LinJie   | M       | 1994-06-12 00:00:00 | 114   |
| 11425   | XieZhou  | M       | 1995-03-11 00:00:00 | 114   |
| 11426   | MingHui  | F       | 1998-08-09 00:00:00 | 114   |
| 11427   | NanNan   | F       | 1995-10-20 00:00:00 | 114   |
+---------+----------+---------+---------------------+-------+

mysql> select * from score;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11425   | 2-271    |     89 |
| 11426   | 1-245    |     61 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

如何汇总？

mysql> select * from score;
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11425   | 2-271    |     89 |
| 11426   | 1-245    |     61 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

mysql> select stu_name,stu_num from student;
+----------+---------+
| stu_name | stu_num |
+----------+---------+
| JiaWei   | 11215   |
| DingQi   | 11328   |
| Baker    | 11422   |
| Bob      | 11423   |
| LinJie   | 11424   |
| XieZhou  | 11425   |
| MingHui  | 11426   |
| NanNan   | 11427   |
+----------+---------+

mysql> select stu_name,cour_num,degree from student,score
    -> where student.stu_num == score.stu_num;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '== score.stu_num' at line 2
mysql> select stu_name,cour_num,degree from student,score
    -> where student.stu_num = score.stu_num;
+----------+----------+--------+
| stu_name | cour_num | degree |
+----------+----------+--------+
| Baker    | 3-105    |     92 |
| Bob      | 1-245    |     84 |
| Bob      | 2-271    |     75 |
| LinJie   | 4-321    |     75 |
| XieZhou  | 2-271    |     89 |
| MingHui  | 1-245    |     61 |
| MingHui  | 2-271    |     82 |
| NanNan   | 1-245    |     78 |
+----------+----------+--------+

这样就对应上了。双等号有错误，改成一个等号。

2.查询所有学生的stu_num，cour_name，degree列：

cour_name在course表中，stu_num与degree在score表中：

mysql> select stu_num,cour_name,degree from score,course
    -> where score.cour_num=course.cour_num;
+---------+-----------+--------+
| stu_num | cour_name | degree |
+---------+-----------+--------+
| 11423   | Math      |     84 |
| 11426   | Math      |     61 |
| 11427   | Math      |     78 |
| 11423   | Circuit   |     75 |
| 11425   | Circuit   |     89 |
| 11426   | Circuit   |     82 |
| 11422   | OS        |     92 |
| 11424   | Bio       |     75 |
+---------+-----------+--------+

先分开查，然后再汇总。

3.查询所有学生的stu_name,cour_name,degree列：

stu_name在表student中，cour_name在course表中，degree在score中。

mysql> select stu_name,cour_name,degree from student,course,score
    -> where student.stu_num=score.stu_num and course.cour_num=score.cour_num;
+----------+-----------+--------+
| stu_name | cour_name | degree |
+----------+-----------+--------+
| Bob      | Math      |     84 |
| MingHui  | Math      |     61 |
| NanNan   | Math      |     78 |
| Bob      | Circuit   |     75 |
| XieZhou  | Circuit   |     89 |
| MingHui  | Circuit   |     82 |
| Baker    | OS        |     92 |
| LinJie   | Bio       |     75 |
+----------+-----------+--------+

通过表中某些共有的字段进行连接。

4.查询114班学生每门课的平均分：

mysql> select cour_num,avg(degree)
    -> from score
    -> where stu_num in (select stu_num from student where class='114')
    -> group by cour_num;
+----------+-------------+
| cour_num | avg(degree) |
+----------+-------------+
| 3-105    |     92.0000 |
| 1-245    |     74.3333 |
| 2-271    |     82.0000 |
| 4-321    |     75.0000 |
+----------+-------------+

 5.查询课程1-245的成绩高于11426同学课程1-245成绩的所有同学的记录：

查出11426号同学1-245课程的成绩：

mysql> select degree from score where stu_num=11426 and cour_num='1-245';
+--------+
| degree |
+--------+
|     61 |
+--------+

查询成绩大于这位同学的记录：

mysql> select * from score where degree>(select degree from score where stu_num=11426 and cour_num='1-245');
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11425   | 2-271    |     89 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

筛选课程号：

mysql> select * from score where degree>(select degree from score where stu_num=11426 and cour_num='1-245') and cour_num='1-245';
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11423   | 1-245    |     84 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

6.查询成绩高于11426同学课程1-245成绩的所有同学的记录：

mysql> select * from score where degree>(select degree from score where stu_num=11426 and cour_num='1-245');
+---------+----------+--------+
| stu_num | cour_num | degree |
+---------+----------+--------+
| 11422   | 3-105    |     92 |
| 11423   | 1-245    |     84 |
| 11423   | 2-271    |     75 |
| 11424   | 4-321    |     75 |
| 11425   | 2-271    |     89 |
| 11426   | 2-271    |     82 |
| 11427   | 1-245    |     78 |
+---------+----------+--------+

子查询就按部就班的依次查询，然后进行嵌套。