我如何 sql 透视列

Question

我看过很多关于数据透视表的问题 tables.Even 如果他们不知道他们在问数据透视表，......

什么是枢轴？
我如何旋转？
我如何旋转这些数据？


+---------+----------+----------+
| Subject | Semester | Attendee |
+---------+----------+----------+
| ITB001  |        1 | John     |
| ITB001  |        1 | Bob      |
| ITB001  |        1 | Mickey   |
| ITB001  |        2 | Jenny    |
| ITB001  |        2 | James    |
| MKB114  |        1 | John     |
| MKB114  |        1 | Erica    |
+---------+----------+----------+

至像这样

+----------+----------+----------------+
| Attendee | Semester | Subject        |
+----------+----------+----------------+
| John     |        1 | ITB001,MKB114  |
| Bob      |        1 | ITB001         |
+----------+----------+----------------+

Answer 1

这根本不是一个数据透视表，它是字符串聚合。

在 SQL Server 2017+ 或 Azure SQL 数据库中：

SELECT Attendee, Semester, Subjects = STRING_AGG(Subject, ',')
  FROM dbo.SomeTableName
  GROUP BY Attendee, Semester;

在较旧和不受支持的版本上，它更丑陋得多，更不用说效率低下了：

SELECT ost.Attendee, ost.Semester, Subjects = STUFF((
  SELECT ',' + ist.Subject
    FROM dbo.SomeTableName AS ist
    WHERE ist.Attendee = ost.Attendee
      AND ist.Semester = ost.Semester
    FOR XML PATH(''), 
    TYPE).value(N'./text()[1]', N'varchar(max)'), 1, 1, '')
FROM dbo.SomeTableName AS ost
GROUP BY ost.Attendee, ost.Semester;

示例db<>fiddle

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