Question

I have 2 tables:

Message:
    id:         INT
    created_at: DATETIME

Comment:
    id:         INT
    message_id: INT
    created_at: DATETIME

One Message has many Comments.

I want to get all Messages ordered by the most recent activity:

• If Message has Comments, then the most recent Comment‘s created_at is used as the Message activity indicator
• If Message doesn’t have Comments, it’s created_at value is the activity indicator

So basically, I want to sort it like a classic e-mail or private messaging system.

Maybe I can INNER JOIN the Comments, but I don’t think it’s necessary to get all the Comments just for the sort.

Also, I thought about creating a column in Message to save the last activity date and update it whenever a Comment is created, but I’d like to see if you have better solutions..

I’m using Doctrine, so if you have a Doctrine-based solution I’d rather that,

Thanks!

Answer 1

What I would do in MySQL is something like:

SELECT *
FROM Messages AS m
ORDER BY GREATEST(
   m.created_at, 
   (SELECT MAX( c.created_at ) FROM Comment AS c WHERE c.message_id = m.id )
) DESC