Question

I have 2 tables. users and table2.
In table2 it can have an entry like

id = 1
right_user_id = 1
left_user_id = 2

in the user table it is possible that there is NO user with id 2.
That means, that the following SQL statement would NOT return the table2-entry with id 1.

//php
$sql = 'SELECT t2.*, u.username AS u_name
        FROM users u, table2 t2
        WHERE t2.right_user_id = '.$user_id.'
              AND t2.left_user_id = u.user_id
        ORDER BY t2.time DESC';

But I would like to get the t2.* info of the entry with id = 1 anyway and u_name should state “(Unknown)”.

What kind of JOIN would I need and how can I do that JOIN?

Answer 1

SELECT t2.*, case 
               when u.username is null 
               then 'Unknown' 
               else u.username 
             end AS u_name      
FROM table2 t2
left outer join users u on t2.left_user_id = u.user_id 
where t2.right_user_id = '.$user_id.'     
ORDER BY t2.time DESC;