我正在学习《数据库系统概念》第六版这本书，它使用了一个大学数据库的例子。这个数据库的模式如下。粗体表示主键：

department(dept_name, building, budget)
course(course_id, title, dept_name, credits)
instructor(ID, name, dept_name, salary)
section(course_id, sec_id, semester, year, building, room number, time_slot_id)
teaches(ID, course_id, sec_id, semester, year)
prereq(course_id, prereq_id)

书中提出了以下问题：
“列出教员的姓名以及他们所教授课程的名称”。
这本书给出的答案是：

select name, title
from instructor natural join teaches, course
where teaches.course id = course.course id;

此外，作者还说：
“相反，以下sql查询计算的结果不同：

select name, title
from instructor natural join teaches natural join course;

要了解原因，请注意讲师和教师的自然连接包含属性（id、name、dept name、salary、course id、sec id），而课程关系包含属性（course id、title、dept name、credits）。因此，这两者的自然连接除了要求course id值相同外，还要求两个输入的dept name属性值相同。然后，此查询将省略所有（讲师姓名、课程名称）对，其中讲师在讲师自己的部门以外的部门教授课程。另一方面，前面的查询正确地输出了这样的对。”
现在我试着用这两个问题来找出区别。

mysql> select * from instructor;
+-------+------------+------------+----------+
| ID    | name       | dept_name  | salary   |
+-------+------------+------------+----------+
| 10101 | Srinivasan | Comp. Sci. | 65000.00 |
| 12121 | Wu         | Finance    | 90000.00 |
| 15151 | Mozart     | Music      | 40000.00 |
| 22222 | Einstein   | Physics    | 95000.00 |
| 32343 | El Said    | History    | 60000.00 |
| 33456 | Gold       | Physics    | 87000.00 |
| 45565 | Katz       | Comp. Sci. | 75000.00 |
| 58583 | Califieri  | History    | 62000.00 |
| 76543 | Singh      | Finance    | 80000.00 |
| 76766 | Crick      | Biology    | 72000.00 |
| 83821 | Brandt     | Comp. Sci. | 92000.00 |
| 98345 | Kim        | Elec. Eng. | 80000.00 |
+-------+------------+------------+----------+
12 rows in set (0.00 sec)

mysql> select * from course;
+-----------+----------------------------+------------+---------+
| course_id | title                      | dept_name  | credits |
+-----------+----------------------------+------------+---------+
| BIO-101   | Intro. to Biology          | Biology    |       4 |
| BIO-301   | Genetics                   | Biology    |       4 |
| BIO-399   | Computational Biology      | Biology    |       3 |
| CS-101    | Intro. to Computer Science | Comp. Sci. |       4 |
| CS-190    | Game Design                | Comp. Sci. |       4 |
| CS-315    | Robotica                   | Comp. Sci. |       3 |
| CS-319    | Image Processing           | Comp. Sci. |       3 |
| CS-347    | Database System Concepts   | Comp. Sci. |       3 |
| EE-181    | Intro. to Digital Systems  | Elec. Eng. |       3 |
| FIN-201   | Investment Banking         | Finance    |       3 |
| HIS-351   | World History              | History    |       3 |
| MU-199    | Music Video Production     | Music      |       3 |
| PHY-101   | Physical Principles        | Physics    |       4 |
+-----------+----------------------------+------------+---------+

mysql> select * from teaches;
+-------+-----------+--------+----------+------+
| ID    | course_id | sec_id | semester | year |
+-------+-----------+--------+----------+------+
| 76766 | BIO-101   | 1      | Summer   | 2009 |
| 76766 | BIO-301   | 1      | Summer   | 2010 |
| 10101 | CS-101    | 1      | Fall     | 2009 |
| 45565 | CS-101    | 1      | Spring   | 2010 |
| 83821 | CS-190    | 1      | Spring   | 2009 |
| 83821 | CS-190    | 2      | Spring   | 2009 |
| 10101 | CS-315    | 1      | Spring   | 2010 |
| 45565 | CS-319    | 1      | Spring   | 2010 |
| 83821 | CS-319    | 2      | Spring   | 2010 |
| 10101 | CS-347    | 1      | Fall     | 2009 |
| 98345 | EE-181    | 1      | Spring   | 2009 |
| 12121 | FIN-201   | 1      | Spring   | 2010 |
| 32343 | HIS-351   | 1      | Spring   | 2010 |
| 15151 | MU-199    | 1      | Spring   | 2010 |
| 22222 | PHY-101   | 1      | Fall     | 2009 |
+-------+-----------+--------+----------+------+
15 rows in set (0.00 sec)

mysql> select name, title
-> from instructor natural join teaches, course
-> where teaches.course_id = course.course_id;
+------------+----------------------------+
| name       | title                      |
+------------+----------------------------+
| Crick      | Intro. to Biology          |
| Crick      | Genetics                   |
| Srinivasan | Intro. to Computer Science |
| Katz       | Intro. to Computer Science |
| Brandt     | Game Design                |
| Brandt     | Game Design                |
| Srinivasan | Robotica                   |
| Katz       | Image Processing           |
| Brandt     | Image Processing           |
| Srinivasan | Database System Concepts   |
| Kim        | Intro. to Digital Systems  |
| Wu         | Investment Banking         |
| El Said    | World History              |
| Mozart     | Music Video Production     |
| Einstein   | Physical Principles        |
+------------+----------------------------+
15 rows in set (0.00 sec)

mysql> select name, title
    -> from instructor natural join teaches natural join course;
+------------+----------------------------+
| name       | title                      |
+------------+----------------------------+
| Crick      | Intro. to Biology          |
| Crick      | Genetics                   |
| Srinivasan | Intro. to Computer Science |
| Katz       | Intro. to Computer Science |
| Brandt     | Game Design                |
| Brandt     | Game Design                |
| Srinivasan | Robotica                   |
| Katz       | Image Processing           |
| Brandt     | Image Processing           |
| Srinivasan | Database System Concepts   |
| Kim        | Intro. to Digital Systems  |
| Wu         | Investment Banking         |
| El Said    | World History              |
| Mozart     | Music Video Production     |
| Einstein   | Physical Principles        |
+------------+----------------------------+
15 rows in set (0.00 sec)

从结果我看不出有什么不同，而且我不明白为什么会有不同。与书中所述不同，讲师和教师的自然连接包含属性id、name、depart name、salary、course id、sec id、sement、year。