How to return a boolean response for a unique and new entry
If I understand your question correctly, you want to select a boolean value based on whether another query (that determines if there are any new entries in the table based on some new flag) selects any rows.

Someone could probably do this in a more optimised way, but it could be achieved by doing this:

SELECT CASE
           WHEN COUNT(y.class) &gt; 0 THEN
               1
           ELSE
               0
       END
FROM (SELECT class
    FROM Features
    WHERE account_username = 'someusername'
    GROUP BY class) y
    LEFT JOIN
    (
        SELECT class
        FROM Features
        WHERE new = 0 AND account_username = 'someusername'
        GROUP BY class
    ) x
        ON x.class = y.class
WHERE x.class IS NULL


This selects 1 if the inner query selects any rows, otherwise 0. The inner query selects the rows that have a new flag associated with them by performing a left join on all the possible classes and all the classes where the flag is 0 where the x.class is null. 

To run it on a single username, create a stored procedure and parameterise the username column. 

Hope this helps!