構建SQL查詢與多個WHERE要求，一個有點複雜，失去了

Question

我工作的一門功課的問題，其中

我需要輪詢三個表並找出誰擁有 小於3.5 GPA學生的名字並正在採取不到9學分

我已經想出瞭如何填充少於9學分的學生的名字，但我堅持計算出他們的學分。

的學生表包含Student ID（主鍵），Name，Year（新鮮，SOPH等），Major，和GPA。

的場表包含course ID（主鍵），Name of class和credit hour rating for that class。

的招生表包含Student ID（使這個外鍵），Course1（FK），Course2，Course3和Course4。

我不是在尋找'正確'的答案，而是一些方向感。

我的查詢到目前爲止

SELECT DISTINCT NAME 
From StudentTable,EnrollmentTable,CourseTable 
WHERE (GPA <3.5) 

我想我需要的東西，GPA檢查後，分析EnrollmentTable爲每一位學生，檢查每個課程（Course1，Course2等），如果NULL！ ，使用該courseID並輪詢課程表以查找該課程的學分值。然後使用SUM檢查以查看是否< 9.

想法？感謝您的幫助！

Answer 1

，你需要獲得的學分總和對於一個給定的學生，這將導致您

讓我們打破它考慮一組。首先，你需要< 3的學生。5 GPA，這是簡單的只是：

SELECT S.Id, S.Name 
FROM Student S 
WHERE S.GPA < 3.5 

它做了一個有點棘手，通過使每個學生在入學的招生多列，而不是多個行（這是比較正常的）

就個人而言，我想將其轉換成一個 '規範化' 的觀點：

SELECT StudentId, Course1 AS CourseId 
FROM Enrollment E 
UNION 
SELECT StudentId, Course2 AS CourseId 
FROM Enrollment E 
UNION 
SELECT StudentId, Course3 AS CourseId 
FROM Enrollment E 
UNION 
SELECT StudentId, Course4 AS CourseId 
FROM Enrollment E 

再加上第一查詢：

SELECT S.Id, S.Name 
FROM Student S 
JOIN 
(
    SELECT StudentId, Course1 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course2 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course3 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course4 AS CourseId 
    FROM Enrollment E 
) E 
    ON E.StudentId = S.StudentId 
WHERE S.GPA < 3.5 

然後，你需要實際的課程，以獲得CreditHourRating：

SELECT S.Id, S.Name, C.CreditHourRating 
FROM Student S 
JOIN 
(
    SELECT StudentId, Course1 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course2 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course3 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course4 AS CourseId 
    FROM Enrollment E 
) E 
    ON E.StudentId = S.StudentId 
JOIN Course C 
    ON E.CourseId = C.CourseId 
WHERE S.GPA < 3.5 

然後你只需要合計爲每一位學生，因此該查詢：

SELECT S.Id, S.Name, SUM(C.CreditHourRating) 
FROM Student S 
JOIN 
(
    SELECT StudentId, Course1 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course2 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course3 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course4 AS CourseId 
    FROM Enrollment E 
) E 
    ON E.StudentId = S.StudentId 
JOIN Course C 
    ON E.CourseId = C.CourseId 
WHERE S.GPA < 3.5 
GROUP BY S.Id, S.Name 

最後，你就具備的條件信用小時收費：

SELECT S.Id, S.Name, SUM(C.CreditHourRating) 
FROM Student S 
JOIN 
(
    SELECT StudentId, Course1 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course2 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course3 AS CourseId 
    FROM Enrollment E 
    UNION 
    SELECT StudentId, Course4 AS CourseId 
    FROM Enrollment E 
) E 
    ON E.StudentId = S.StudentId 
JOIN Course C 
    ON E.CourseId = C.CourseId 
WHERE S.GPA < 3.5 
GROUP BY S.Id, S.Name 
HAVING SUM(C.CreditHourRating) < 9 

Answer 2

如果數據庫模式取決於你，那麼我會改變它一點。招生代表學生和課程之間的映射。我將它簡化爲兩列：StudentID和CourseID（如果您不希望表的組合主鍵，則可以添加EnrollmentID的第三列）。

完成後，您可以加入Students - > Enrollments - > Courses。按StudentID分組，並使用HAVING子句對學生的學分和學生GPA中的WHERE進行篩選。

有意義嗎？