Question

I’m working on a hobby program and figure there is a better way of doing this. What I have is this query here:

SELECT username
FROM users

Which returns ALL possible matches.
I then run this query:

SELECT username
FROM alreadyDone
WHERE Name='test'

Which gets me the matches that we have completed. I then just add these all to arraylists in Java and then remove the same ones through the collections interface.

I figure there is a way to get this done all though MySQL, and playing around with various joins and such, I cannot figure out which way to go. Can someone point me in the right direction?

Answer 1

If I understand correctly, you’re trying to find those users who are NOT in the alreadyDone table for the ‘test’.

SELECT u.username
    FROM users u
    WHERE NOT EXISTS(SELECT 1
                         FROM alreadyDone ad
                         WHERE ad.username = u.username
                             AND Name = 'test');

Alternatively, you could also write this as:

SELECT u.username
    FROM users u
        LEFT JOIN alreadyDone ad
            ON u.username = ad.username
                AND Name = 'test'
    WHERE ad.username IS NULL;