MySQL从多个表中以特定顺序选择行

Question

I have 2 tables in a mysql database ("comments" and "replies")

They have the schemas:

comments: id,userId,text
replies: id,userId,commentId,text

I need a mysql query that will fetch all comments from the comment table, and after each comment, the corresponding replies from the replies table...

so if we had: [comment 1] and [comment 2] in the comments table, and [reply 1] (to comment 1) and [reply 2] (to comment2) in the replies table

then it would return:

    [comment 1]
    [reply 1]
    [comment 2]
    [reply 2]

Answer 1

You would need to join the two tables & then order based on the commentID & replyID for multiple replies.

In the following I have added a dummy replyID of 0 for the original comment. The tables are joined using UNION ALL. Note that I have renamed the original id column of the comments table & reply id so they do not clash.

SELECT id commentID, userID, text, 0 replyID FROM test.comments
UNION ALL
SELECT commentID, userID, text, id replyID FROM test.replies
ORDER BY commentID, replyID;