让学生的课程在同一行的查询

Question

我需要一个查询来获取获得课程 1、课程 2 和课程 3 的学生。我可以这样查询：

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'))

但我需要这样的结果集：姓名、姓氏、电子邮件、课程 1、课程 2、课程 3。一个学生没有多行，一行，如果学生获得该课程，则写课程名称。

事实上，我可以想象如何编写该查询（使用子选择），想知道更好的选择。

Answer 1

您可以使用 PIVOT table 运算符来执行此操作，如下所示：

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

请注意， 由于您没有其他列显示在每个学生的每列课程下，此查询将为每门课程显示 CourseID 值姓名，null 如果学生没有这门课程。您应该选择显示更合适的列，例如标记：

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

这会给你：

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

另请注意，您应该注意在锚查询中选择的列：

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

因为 PIVOT table 运算符将按除您列出并用于透视的列之外的所有列进行分组：

MAX(CourseID)
  FOR CourseName IN

因此在您的情况下，它将按 firstname, lastname, Email.

分组

Answer 2

我认为您可以使用比您更好的查询并使用 MAX(CASE ...) 来实现您的预期结果，如下所示：

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;

让学生的课程在同一行的查询

Query that gets students with their courses in same row

sql

tsql

sql-server

pivot