GROUP BY 并将列值放入单行列

Question

我在想是否可能获得使用 GROUP BY 房间的所有价值。 例如，我有一个名为主题、房间和教室的表

表主题

   subject_id | subject_name
    -------------------
        1     | math
        2     | science
        3     | english

桌房

    room_id | room_name
   -------------------
       1    | sunflower
       2    | cornflower
       3    | redflower

桌子教室

    id | room_id | subject_id 
   -------------------
    1  | 1       | 1          
    2  | 1       | 2
    3  | 1       | 3

现在查询。 不确定我使用的查询是否正确或足够的查询..但输出的查询显示预期不是

SELECT * FROM classroom 
LEFT JOIN subject ON classroom.subject_id = subject.subject_id
LEFT JOIN room ON classroom.room_id = room.room_id 
GROUP BY room_name

现在我想像这样期待 output

课堂

room_name | subject_name 
------------------------
sunflower | math
          | science
          | english

它应该按向日葵分组为一行

Answer 1

GROUP BY旨在减少行数并为这些行启用聚合。 在您的情况下，以下查询，没有group by产生：

SELECT room_name, subject_name
FROM classroom 
LEFT JOIN subject ON classroom.subject_id = subject.subject_id
LEFT JOIN room ON classroom.room_id = room.room_id 

+-----------+--------------+
| room_name | subject_name |
+-----------+--------------+
| sunflower | english      |
| sunflower | math         |
| sunflower | science      |
+-----------+--------------+

所以使用GROUP BY没有多大意义，但如果我们这样做，结果如下：

SELECT room_name, subject_name
FROM classroom 
LEFT JOIN subject ON classroom.subject_id = subject.subject_id
LEFT JOIN room ON classroom.room_id = room.room_id 
GROUP BY room_name, subject_name

+-----------+--------------+
| room_name | subject_name |
+-----------+--------------+
| sunflower | english      |
| sunflower | math         |
| sunflower | science      |
+-----------+--------------+

请注意，每行的值都显示在两列中。 这既是预期的，也是期望的。

GROUP BY不是一个“表示”工具，它会抑制第一列中的值的 output，这是一个“报告工具”或“表示层”应该处理的东西。

演示