MySQL一对多加入Group By只返回一个观察值

Question

我有一个注释表和一个标记表。对于每条评论，可能有多个标签，或者没有。我想加入这两个，所以我可以得到每个评论的标签列表。

CommentTable：

+---------+----------+---+ 
|CommentID| Title | ..| 
+---------+----------+---+ 
| 1  | animals| | 
| 2  | plants | | 
+---------+----------+---+ 

TagTable：

+---------+----------+---+ 
| TagID |CommentID | ..| 
+---------+----------+---+ 
| 5 |  1 | | 
| 6 |  1 | | 
| 7 |  3 | | 
+---------+----------+---+ 

因此，查询应返回代码，（5,6），用于一个commentID == 1和空数组为CommentID == 2

这是我 - 只选择最后一个ID，而不是倍数：

SELECT c.CommentID, c.Title, t.TagID FROM Comment as c 
     LEFT OUTER JOIN Tag as t ON c.CommentID = t.CommentID 
     GROUP BY t.TagID 

Answer 1

您可以使用GROUP_CONCAT将多行中的数据转换为单个分隔字符串：

SELECT a.CommentID, 
      a.Title, 
      GROUP_CONCAT(b.TagID ORDER BY b.TagID) AS tags 
FROM  CommentTable a 
LEFT JOIN TagTable b ON a.CommentID = b.CommentID 
GROUP BY a.CommentID, 
      a.Title 

在这种情况下，如果评论没有相应的标记，那么该字段将仅为NULL。

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SQLFiddle Demo

Answer 2

试试这个：

SELECT c.CommentID, c.Title, t.TagID FROM Comment as c 
     LEFT OUTER JOIN Tag as t ON c.CommentID = t.CommentID 

EDIT1：如果你想返回每组只有一行按照注释

SELECT c.CommentID, c.Title,MAX(t.TagID) 
FROM Comment as c 
left OUTER JOIN TagTable as t ON c.CommentID = t.CommentID 
GROUP BY c.CommentID, c.Title 

Answer 3

你不要为这种情况需要组：

SELECT c.CommentID, c.Title, t.TagID FROM Comment as c 
LEFT OUTER JOIN Tag as t ON c.CommentID = t.CommentID