CODEWITHSUNDEEP
mysqlsql
Bogdan Zurac
Bogdan Zurac

Reputation: 6451

Query parents and children in self-referencing table

I have a Comments table like below, in MySQL:

content           created_at                 id  parent_id
"second comment", 2014-06-03T10:08:44+0000,  37,      -1
"third comment",  2014-06-03T10:10:35+0000,  40,      -1
"Under third",    2014-06-03T10:10:44+0000,  41,      40
"Under second",   2014-06-03T10:11:59+0000,  42,      37

Users can add new comments, which won't have a parent_id, as they are not children of other comments; users can also reply to comments that have been added through the previous method, so they are children of the primary comments, like on a second level of hierarchy. The parent_id column represents the id of a parent comment, if present. If the comment doesn't have a parent, default parent_id is -1.

That being said, I would like to query all the comments from the table, each parent followed by its children, ordered by created_at ASC. Example from the above data set:

second comment
Under second
third comment
Under third

I thought of using GROUP BY, as it resembles a grouping strategy, but not actually grouping all children into a single row. What would be a solution to this kind of query ? Are there more types of solutions ?

Upvotes: 3

Views: 1707

Answers (1)

Martin K.
Martin K.

Reputation: 1060

It didn't test but i think this should work in MySQL too:

ORDER BY CASE WHEN parent_id=-1 THEN id ELSE parent_id END, created_at

Edit: If you cannot assume the Ids to be ascending in the same logical order as the Comments, it get's a little more complex:

SELECT parent_id,id,created_at parent_date,null child_date,content
    FROM Comments
    WHERE parent_id=-1
UNION
SELECT c.parent_id,c.id,p.created_at as parent_date,c.created_at as child_date,c.content
FROM Comments c
    JOIN (SELECT id,created_at,content
            FROM Comments
            WHERE parent_id=-1
            GROUP BY id,created_at,content) p ON p.id=c.parent_id
ORDER BY parent_date,child_date

Upvotes: 3

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