Question

In MySql I have table:

idPost | idPostParent | postText                              | dateCreated         |
1      | 1            |  This is parent post no1              | 2012-01-01 12:00:00 |
2      | 2            |  This is parent post no2              | 2012-01-02 13:00:00 |
3      | 1            |  This is first reply on idPost:1      | 2012-01-02 13:30:00 | 
4      | 4            |  This is parent post no3              | 2012-01-04 10:00:00 |
5      | 2            |  This is first reply on idPost:2      | 2012-01-04 11:00:00 |
6      | 1            |  This is second reply on idPost:1     | 2012-01-05 15:00:00 |
7      | 2            |  This is second reply on idPost:2     | 2012-01-06 17:00:00 |

NOTE:

1. PARENT post is detected if idPost = idPostParent
2. REPLY post is detedted if idPost <> idPostParent and reply is
   related to post writen in idPostParent

I need SQL query for sorting and grouping record sets in table by some rules:

1. Posts must be grouped by idPostParent (parent post first then its
   replies)
2. The PARENT post that is latest entered OR have latest entered reply must be on the top.

The resultset should be:

idPost | idPostParent | postText                              | dateCreated         |
2      | 2            |  This is parent post no2              | 2012-01-02 13:00:00 |
5      | 2            |  This is first reply on idPost:2      | 2012-01-04 11:00:00 |
7      | 2            |  This is second reply on idPost:2     | 2012-01-06 17:00:00 |
1      | 1            |  This is parent post no1              | 2012-01-01 12:00:00 |
3      | 1            |  This is first reply on idPost:1      | 2012-01-02 13:30:00 |
6      | 1            |  This is second reply on idPost:1     | 2012-01-05 15:00:00 |
4      | 4            |  This is parent post no3              | 2012-01-04 10:00:00 |

REASON:
The PARENT post idPost=2 has latest entered reply post so its thread must be on the top of the table.

Thanks to anyone who may contribute to the solution!

Answer 1

Here’s one way:

SELECT p.*
FROM posts p
LEFT JOIN
 (SELECT idPostParent, MAX(dateCreated) AS time_of_newest
  FROM posts
  GROUP BY idPostParent) p2
 ON p.idPostParent = p2.idPostParent
ORDER BY time_of_newest DESC, idPost=p.idPostParent DESC, dateCreated DESC

Explanation:

The sub-query

SELECT idPostParent, MAX(dateCreated) AS time_of_newest
  FROM posts
  GROUP BY idPostParent

selects the maximum date created per “thread” (idPostParent).

This is LEFT JOINd to the main posts table, so that every thread has a time_of_newest for that thread.

We sort by time_of_newest DESCENDING (this puts thread 2 in front of thread 1 in front of thread 4).

Then, we sort by idPost=idPostParent1 DESC in order to get parent posts in front of replies (parent posts have a 1 here and replies all have a 0, and since we sort descending parent posts go first).

Finally, we sort by dateCreated DESC to get the replies in order.