MySQL one-to-many join with Group By only returns one observation

Question

I have a comment table and a tag table. For each comment, there could be multiple tags, or none. I want to join the two so I can get a list of tags for each comment.

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Answer 1

You can use GROUP_CONCAT to turn data in multiple rows into a single delimited string:

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

In this case, if a comment does not have a corresponding tag, the field would just be NULL.

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SQLFiddle Demo