Question

dHello can you help me with a query about ordering the topics by most recent posts? i mean that i want a topic to go on top when someone post on it. Here are the tables:

Discussion

• id_discution
• name
• id_category
• date

Comments

• id_comment
• id_user
• description
• type
• id_discussion
• date (timestamp)

here is my code but i get duplicates of topics but they are orderd good

select discussion.id_discution, discussion.name, discussion.id_category, 
    discussion.date, discussion.report, comments.id_comment, comments.id_pet_dem_des, 
    comments.date,comments.type 
from discussion,comments 
where discussion.id_discution=comments.id_discussion 
    AND type=3
order by comments.id_comment  desc

Answer 1

GROUP BY the discussion table’s primary key…

SELECT
  discussion.id_discution, discussion.name, discussion.id_category, discussion.date, discussion.report
FROM
  discussion
INNER JOIN
  comments
    ON  discussion.id_discution = comments.id_discussion
    AND type=3
GROUP BY
  discussion.id_discution
ORDER BY
  MAX(comments.date)  desc 

Note: I don’t include any data from the comments table in the SELECT clause.

This is because you only want each discussion to appear once, but each discussion can have many comments. You either have the discussion appear many times, and include the comments, or you don’t include the comments data in the results.

You could include aggregate values, such as COUNT(*) to show how many comments there are MAX(comments.date) to show how recently the discussion was commented on, etc, etc.