Mysql 在自然连接上使用WHERE子句
我正在学习《数据库系统概念》第六版,该书以一所大学的数据库为例。 该数据库的架构如下所示。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) 此外,作者还说: “相比之下,以下SQL查询计算的结果不同:Mysql 在自然连接上使用WHERE子句,mysql,sql,natural-join,Mysql,Sql,Natural Join,我正在学习《数据库系统概念》第六版,该书以一所大学的数据库为例。 该数据库的架构如下所示。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_s
选择名称、标题
从讲师自然连接到教授自然连接课程;
要了解原因,请注意讲师和讲师的自然连接包含属性
(ID、名称、部门名称、薪资、课程ID、sec ID),而课程关系包含
属性(课程id、标题、系名、学分)。因此,这些属性的自然连接
two将要求来自两个输入的dept name属性值为
相同,除了要求课程id值相同之外。此查询
然后将省略所有(讲师姓名、课程名称)对,其中讲师
在讲师所在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些对。”
现在,我尝试了两种查询,以了解其中的差异
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)
从结果来看,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、名称、部门名称、薪水、课程ID、sec ID、学期、年份 这本书是正确的。这些查询不是等价的。样本数据(detp_名称值不同): 就个人而言,我不会使用任何建议的查询:
-- mixing natural join with old-comma syntax
select name, title
from instructor natural join teaches, course
where teaches.course_id = course.course_id;
--Output
--name title
--Prof X Course_1
-- natural join(potentially dangerous-depends on names of columns)
select name, title
from instructor natural join teaches natural join course;
-- 0 rows selected
并将其改写为:
SELECT i.name, c.title
FROM instructor i
JOIN teaches t
ON t.ID = i.ID
JOIN course c
ON t.course_id = c.course_id;
您从未显示过
课程
表格的样本数据,但书籍有时会出现打字错误。是的,您是对的。我刚刚编辑了它。@ginos书是对的。我不明白为什么自然连接与第一个显式连接有任何不同。从我所看到的,只有连接列具有相同的名称。不要使用自然连接
!我认为这是可憎的,因为它甚至不使用正确的声明的键连接的外键关系。相反,使用适当、明确、标准的join
语法,使用on
或使用子句。谢谢@Lukasz Szozda
SELECT i.name, c.title
FROM instructor i
JOIN teaches t
ON t.ID = i.ID
JOIN course c
ON t.course_id = c.course_id;