在同一个表中选择与新旧旧不同

Question

I have one table named: thread_comment

Now the table is getting filled as followed:

thread_comment_id   thread_id   user_id   thread_comment       thread_comment_time
       1               2           1     This is a comment     2016-09-14 15:30:28
       2               4           1     This is a comment     2016-09-14 15:32:28
       3               2           1     This is a comment     2016-09-14 15:33:28
       4               5           1     This is a comment     2016-09-14 15:34:28
       5               7           1     This is a comment     2016-09-14 15:35:28
       6               2           1     This is a comment     2016-09-14 15:37:28
       7               2           1     This is a comment     2016-09-14 15:40:28

I want to show the newest threads to my page. for example:

as number one i want thread_comment_id 7 as number two i want thread_comment_id 5

I skipped 6 because i don't want any duplicates in the list.

In order to do so i did the folowing:

$sql = "SELECT DISTINCT thread_id FROM thread_comment ORDER BY thread_comment_time DESC"

This is kind of working (Not showing any duplicates). However the order does not make any sense...

For example:

It goes like 5 - 6 -4 - 7 etc...

Answer 1

The column used in the ORDER BY isn't specified in the DISTINCT . You need to use an aggregate function and GROUP BY to make the DISTINCT work.

SELECT DISTINCT thread_id, max(thread_comment_id) FROM thread_comment GROUP BY thread_id ORDER BY max(thread_comment_id) DESC, thread_id

EDIT: added aggregate func max() Also thread_id is not mandatory in the ORDER BY

Answer 2

It I understand correctly, you want one row per thread. Here is a way to do this:

select tc.*
from thread_comment tc
where tc.thread_date = (select max(tc2.thread_date)
                        from thread_comment tc2
                        where tc2.thread_id = tc.thread_id
                       );