如何編寫正確的查詢以找到當前不在課程中的學生

Question

我正在嘗試找到剩餘的可用於註冊課程的學生。要做到這一點，我需要找到當前活躍並且當前不在課程中的學生。 Bu我認爲我正在寫的查詢是不正確的，因爲當我運行查詢時，它會一直顯示一組空的結果，我怎麼才能找到能夠註冊課程的剩餘學生？

下面是當前查詢：

SELECT st.StudentId, StudentAlias, StudentForename, StudentSurname 
FROM Course c 
     INNER JOIN Student_Course sc 
      ON c.CourseId = sc.CourseId 
     INNER JOIN Student st 
      ON sc.StudentId = st.StudentId 
WHERE (c.CourseId = 1 AND 
     sc.StudentId IS NULL 
     AND st.Active = 1) 
ORDER BY st.StudentAlias 

下面是表：

學生：（所有學生的列表）

StudentId StudentAlias StudentForename StudentSurname Active 
1   u09382  James   Smith   1 
2   u83923  John    Brooks   1 
3   u38292  Karen   Bradshaw  0 
4   u20039  Chris   Cameron   1 
5   u39399  Jane    Fields   1 

課程：（全部名單課程）

CourseId CourseNo CourseName 
1   INFO121 ICT 
2   BUS122 Business and Finance 
3   ENG432 English Language 

Student_Course：（學生名單報名參加相應課程）

StudentId CourseId 
1   1 
1   3 
2   1 
4   2 
5   2 

因此，舉例來說，如果我選擇CourseId = 1，那麼它應該顯示學生誰是積極的，但還沒有明顯不CourseId = 1這意味着它它應該顯示這些學生低於招生的可能性爲課程：

4 - Chris Cameron 
5 - Jane Fields 

Answer 1

試試這個查詢

SELECT s.studentId, s.Forename 
FROM Student s 
WHERE s.studentId NOT IN (SELECT studentId 
FROM Student_Course c WHERE c.courseId = 1) AND s.Active = 1 

courseId = 1的內部子查詢返回學生，在外部查詢中我們過濾這些學生。

Answer 2

試試這個

SELECT st.StudentId, StudentAlias, StudentForename, StudentSurname 
FROM Course c 
     INNER JOIN Student_Course sc 
      ON c.CourseId = sc.CourseId 
     INNER JOIN Student st 
      ON sc.StudentId = st.StudentId 
WHERE (sc.CourseId <> 1 
     AND st.Active = 1) 
ORDER BY st.StudentAlias 

Answer 3

要找到活動的學生沒有一門課程，然後是這樣的： -

SELECT st.StudentId, StudentAlias, StudentForename, StudentSurname 
FROM Student st 
LEFT OUTER JOIN Student_Course sc ON sc.StudentId = st.StudentId 
LEFT OUTER JOIN Course c ON c.CourseId = sc.CourseId 
WHERE c.CourseId IS NULL 
AND st.Active = 1 
ORDER BY st.StudentAlias 

如果您要檢查學生不是在一個特定的課程，然後

SELECT st.StudentId, StudentAlias, StudentForename, StudentSurname 
FROM Student st 
LEFT OUTER JOIN Student_Course sc ON sc.StudentId = st.StudentId 
LEFT OUTER JOIN Course c ON c.CourseId = sc.CourseId AND CourseId = 1 
WHERE c.CourseId IS NULL 
AND st.Active = 1 
ORDER BY st.StudentAlias 

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