在同一行中獲取學生課程的查詢

Question

我需要一個查詢來獲取獲得課程1，課程2和課程3的學生。我可以查詢這樣的：

SELECT k.name as firstname, k.surname as lastname, k.Email 
FROM Students k 
JOIN StudentCourses dn ON dn.StudentID = k.StudentID 
WHERE dn.CourseID IN 
(SELECT CourseID FROM Courses WHERE CourseName IN ('Course1','Course2','Course3')) 

但我需要設置一個這樣的結果是：姓名，電子郵件，Course1，Course2，Course3。如果學生獲得該課程，則不會爲學生提供多行，也不會爲一行提供課程名稱。

事實上，我可以想象如何編寫該查詢（與subselects），想知道更好的選擇。

Answer 1

可以使用PIVOT表操作要做到這一點，是這樣的：

WITH CTE 
AS 
(
    SELECT 
    k.name as firstname, 
    k.surname as lastname, 
    k.Email, 
    c.CourseName, c.CourseID 
    FROM Students k 
    JOIN StudentCourses dn ON dn.StudentID = k.StudentID 
    INNER JOIN Courses c ON dn.CourseID = c.CourseID 
    WHERE c.CourseName IN ('Course1','Course2','Course3') 
) 
SELECT * 
FROM CTE AS c 
PIVOT 
(
    MAX(CourseID) 
    FOR CourseName IN ([Course1], [Course2], [Course3]) 
) u; 

• SQL Fiddle Demo

---

需要注意的是，既然你沒有其他在每個學生的每個欄目下顯示的列，此查詢將顯示CourseID VA如果學生沒有這門課程，則爲每個課程名稱提供。你應該選擇以顯示更多相應的列，而不是像馬克例如：

WITH CTE 
AS 
(
    SELECT 
    k.name as firstname, 
    k.surname as lastname, 
    k.Email, 
    c.CourseName, dn.Mark 
    FROM Students k 
    JOIN StudentCourses dn ON dn.StudentID = k.StudentID 
    INNER JOIN Courses c ON dn.CourseID = c.CourseID 
    WHERE c.CourseName IN ('Course1','Course2','Course3') 
) 
SELECT * 
FROM CTE AS c 
PIVOT 
(
    MAX(Mark) 
    FOR CourseName IN ([Course1], [Course2], [Course3]) 
) u; 

• Updated SQL Fiddle Demo

這會給你：

| firstname | lastname | Email | Course1 | Course2 | Course3 | 
|-----------|----------|-------|---------|---------|---------| 
| StudentA |  test | test |  19 |  20 |  15 | 
| StudentB |  test | test |  16 |  17 |  20 | 
| StudentC |  test | test |  20 |  19 |  15 | 

還要注意的是，你應該仔細查看您在錨點查詢中選擇的列：

... 
SELECT 
    k.name as firstname, 
    k.surname as lastname, 
    k.Email, 
    c.CourseName, dn.Mark 
... 

因爲PIVOT表運營商將通過組除了你列出，並用樞列的所有列：

MAX(CourseID) 
    FOR CourseName IN 

所以你的情況，它會按firstname, lastname, Email。

Answer 2

我想你可以使用一個更好的查詢比你和使用MAX(CASE ...)實現這樣您預期的結果：

SELECT k.name as firstname, k.surname as lastname, k.Email, 
    MAX(CASE WHEN c.CourseName = 'Course1' THEN 'Yes' ELSE 'No' END) As Course1, 
    MAX(CASE WHEN c.CourseName = 'Course2' THEN 'Yes' ELSE 'No' END) As Course2, 
    MAX(CASE WHEN c.CourseName = 'Course3' THEN 'Yes' ELSE 'No' END) As Course3 
FROM Students k 
JOIN StudentCourses dn ON dn.StudentID = k.StudentID 
JOIN Courses c ON c.CourseID = dn.CourseID AND c.CourseName Like 'Course[1-3]' 
GROUP BY k.name, k.surname, k.Email;