mysql - 不确定我是否正确执行 sql 查询

Question

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)

student(ID, name, dept_name, tot_cred)

takes(ID, course_id, sec_id, semester, year, grade)

1. 查找超过 50 名学生参加的每门课程的课程名称、学期和年份

   select course.title , takes.semester , takes.year
   from course
   natural join takes
   where course.course_id = takes.course_id
   having count(distinct ID) > 50
   

2. 查找包含多个部分的每门课程的标题

   select title 
   from course
   natural join section 
   where course.course_id = section.course_id
   having count(distinct sec_id) > 1
   

3. 查找在 Comp 中教授超过 5 门课程的所有讲师的 ID。科学。部

   select ID
   from instructor
   natural join course
   where course.dept_name = instructor. dept_name
   having count(credits)>5
   

这也应该是学分或 course_id

4. 查找所有未教授生物系提供的任何模块的教师

这个我什至不知道从哪里开始

Answer 1

select count(t.id), c.title , t.semester , t.year
from course c
left join takes t on t.cource_id=c.cource_id
group by 2,3,4
having count(t.id) > 50

select c.title, count(s.sec_id) 
from course c
left join section  s on c.course_id = s.course_id
group by 1
having count(s.sec_id) > 1

select i.ID, count(c.id)
from instructor i
left join course c on c.dept_name = i. dept_name
group by 1
having count(c.id)>5

select i.ID
from instructor i
left join department d on d.dept_name=i.dept_name
where d.dept_name<>'Biology'

Answer 2

您的所有查询都缺少GROUP BY子句。

如果您使用NATURAL JOIN，则不需要WHERE子句来关联表 -NATURAL JOIN自动执行此操作。

select course.title , takes.semester , takes.year
from course
natural join takes
GROUP BY course.title, takes.semester, takes.year    
having count(distinct ID) > 50

select title 
from course
natural join section 
GROUP BY title
having count(distinct sec_id) > 1

select ID
from instructor
natural join course
GROUP BY ID
having count(credits)>5

您的最后一个查询还有其他几个问题。您没有使用teaches表格将教师链接到课程，也没有检查课程是否在 Comp 中。科学。部。

SELECT i.id
FROM instructor AS i
JOIN teaches AS t ON i.id = t.sec_id
JOIN course AS c ON t.course_id = c.id
WHERE c.dept_name = "Comp. Sci."
HAVING COUNT(*) > 5