真的需要优化我的SQL:选择使用NN表的每一行的所有外行

Question

我有一个NN表,举个例子说教师和主题:

有一张Teacher表如下:

    +---------------+-----------------------+
    |    Id         | Name                  |
    +---------------+-----------------------+
    | 1             | John Williams         |
    | 2             | Erika Miller          |
    +---------------+-----------------------+

有一张Subject表如下:

    +---------------+-----------------------+
    |    Id         | Name                  |
    +---------------+-----------------------+
    | 1             | Music                 |
    | 2             | Art                   |
    | 3             | Languages             |
    | 4             | Geography             |
    +---------------+-----------------------+

还有一个TeacherTeachesSubject如下:

    +---------------+-----------------------+
    |  TeacherId    |        SubjectId      |
    +---------------+-----------------------+
    | 1             | 1                     |
    | 1             | 2                     |
    | 1             | 3                     |
    | 2             | 3                     |
    | 2             | 4                     |
    +---------------+-----------------------+

只有每个表有超过10000个条目.

我需要做一个查询,返回我这样的事情:

+---------------+-----------------------+
|    Teacher    | SubjectsOfThatTeacher |
+---------------+-----------------------+
| John Williams | Music, Art, Languages |
| Erika Miller  | Languages, Geography  |
+---------------+-----------------------+

我现在正在做的是:

1. SELECT * FROM Teachers;

2. 将结果保存在数组中.

3. 在每个循环中迭代执行此查询的数组:

   SELECT Name 
   FROM Subjects inner join "everything" 
   WHERE TeacherTeachesSubject.TeacherId = actualteacherid;
   

4. 将结果保存为String,用逗号分隔.

每次我想从表中选择所有数据时,我都会进行10000次查询.

你知道如何有效地开展这项工作吗？我真的不需要逗号,我只需要它在String中作为String在HTML列中显示.

Answer 1

使用SQL Server 2017,使用STRING_AGG():

SELECT t.Name, STRING_AGG(s.Name, ', ') SubjectsOfThatTeacher
FROM Teacher t
INNER JOIN TeacherTeachesSubject tts ON tts.TeacherId = t.Id
INNER JOIN Subject s ON s.Id = tts.SubjectId
GROUP BY t.Id, t.Name