Question

I’m struggling with this for hours so please help me.
This is my users table

id | username | last_activity(timestamp for online system)

And this is my friends table

id | uid | fid

What I want is to order the output by last_activity
My current query looks like this:

SELECT fid FROM friends WHERE uid='$user_id'

Answer 1

You want to use INNER JOIN to join the two tables:

SELECT f.fid 
FROM friends f 
INNER JOIN users u 
ON u.id = f.id 
ORDER BY u.last_activity DESC

Always make sure you type out a real JOIN clause as some ways are old and getting more and more unsupported.

Read more here:
INNER JOIN on w3schools