SQL：连接多个表

Question

编写一个 sql 查询来选择参加过“JOHN”教授的所有讲座但没有参加过“JOSEPH”教授的任何讲座的学生的姓名。

我已经写了这个查询，但它给了我正在听约瑟夫讲课的学生的名字。

select distinct s.studentname 
from Student s,Attendance a,Lecture l 
where s.StudentId=a.studid 
  and l.lecId = a.lectureid 
  and l.professor='JOHN' 
  and l.professor != 'JOSEPH';

我在上述查询中犯了一些错误，但无法识别。

这是表结构

学生桌

# StudentId, StudentName, Sex
'1', 'AMY', 'M'
'2', 'JACK', 'M'
'3', 'TONY', 'M'
'4', 'TARA', 'M'
'5', 'SARAH', 'F'
'6', 'TOM', 'F'

讲台

# LecId, Subject, Professor
1, MATH, JOHN
2, MATH, JOSEPH
3, PHY, MARK
4, PHY, MAX
5, PHY, JOHN
6, CHEM, JOHN
7, CHEM, JOSEPH
8, HISTORY, JOSEPH

考勤表

# StudId, LectureId
'1', '1'
'1', '2'
'3', '1'
'2', '5'
'2', '6'
'3', '4'
'1', '6'
'4', '5'
'5', '1'
'5', '2'
'1', '3'
'1', '4'
'1', '5'
'1', '6'

Answer 1

您需要查看属于给定学生的所有行，因此简单的where 子句无法满足您的要求。相反，您可以使用带有 having 子句的聚合和文件管理器：

select s.studentname 
from student s
inner join attendance a on s.studentId = a.studid
inner join lecture l on l.lecId = a.lectureid 
group by s.studentId, s.studentname 
having max(l.professor= 'JOHN') = 1 and max(l.professor = 'JOSEPH') = 0

如果您希望教师添加所有约翰的讲座，而不是约瑟夫的讲座，那么：

select s.studentname 
from student s
inner join attendance a on s.studentId = a.studid
inner join lecture l on l.lecId = a.lectureid 
group by s.studentId, s.studentname 
having 
    sum(l.professor= 'JOHN') = (select count(*) from lecture where professor = 'JOHN')
    and max(l.professor = 'JOSEPH') = 0

Answer 2

如果你的 MySQL 版本支持 GROUP_CONCAT，你可以试试下面的脚本-

Demo Here

SELECT S.StudentId,S.StudentName, GROUP_CONCAT(DISTINCT L.Professor)
FROM Attendance A
INNER JOIN Lecture L ON A.LectureId = L.LecId
INNER JOIN Student S ON A.StudId = S.StudentId
GROUP BY S.StudentId,S.StudentName
HAVING  GROUP_CONCAT(DISTINCT L.Professor) = 'JOHN'

以上查询将返回仅从“JOHN”学习课程的学生列表。没有其他人。

但如果您的要求更改为在列表中添加更多教授，您可以在 HAVING 子句检查中相应地添加教授名称。但是您需要在这种情况下应用适当的排序。

Answer 3

试试：

select * from Student st
where exists(select * from Lecture l
             join Attendance a
             on l.LecId = a.LectureId
             where l.Professor = 'John'
             and a.StudId = st.StudentId)
and exists(select * from Lecture l
               join Attendance a
               on l.LecId = a.LectureId
               where l.Professor = 'Joseph'
               and a.StudId = st.StudentId)

Answer 4

select s.StudentName 
from Student s
inner join Attendance a on s.StudentId = a.StudId
inner join Lecture l on l.LecId = a.LectureId 
group by s.StudentId, s.StudentName 
having 
    sum(l.Professor= 'JOHN') = (select count(*) from lecture where Professor = 'JOHN')
    and max(l.Professor = 'JOSEPH') = 0
order by s.StudentName

#asked in interview (MakeMyTrip)

Answer 5

我重新表述了你所做的加入...但它似乎有效。

此查询是否返回您要查找的数据集？

SELECT s.studentname, l.Subject, l.Professor
FROM Student s JOIN Attendance a
ON s.StudentId = a.StudId
JOIN Lecture l 
ON l.lecId = a.lectureid
WHERE l.professor='JOHN' 
  AND l.professor != 'JOSEPH';

Answer 6

MSSQL：

WITH cte AS
(
    SELECT StudentName, ROW_NUMBER() 
    OVER(
        PARTITION BY studid
        ORDER BY studid
    ) cou 
    FROM attendance a,lecture l,student s 
    WHERE a.LectureId = l.LecId
    AND a.StudId = s.StudentId
    AND l.Professor = 'john'
)
SELECT StudentName
FROM cte
WHERE cou = 3

Except

SELECT StudentName
FROM attendance a, lecture l, student s 
WHERE a.LectureId = l.LecId
AND a.StudId = s.StudentId
AND l.Professor = 'joseph'