SET @time = '2011-27-10 14:00:00';
SET @stud = 4;

SELECT id_teacher
FROM bb_appointment root
WHERE id_student = @stud
AND id_teacher NOT IN (
    SELECT id_teacher
    FROM bb_appointment
    WHERE time = @time
--) AND 3 > (
--    SELECT count(*) AS hours
--    FROM (
--        SELECT id_teacher
--        FROM bb_appointment
--        WHERE id_teacher = root.id_teacher
--    ) as lambda
) AND id_teacher NOT IN (       -- this is definitely the last resort.
    SELECT id_teacher           -- TODO: make a better solution faggot
    FROM (
        SELECT id_teacher, count(*) AS hours
        FROM (
            SELECT id_teacher   -- select all teachers work events of today
            FROM bb_appointment
        ) as lambda
        GROUP BY id_teacher     -- calculate total hours for each teacher
        ORDER BY hours ASC
    ) as lambda
    WHERE hours > 2             -- and reject any results with more than two hours
);

-- TODO: Merge these somehow

SELECT id                       -- START SELECTING LEAST ACTIVE TEACHER TODAY
FROM (
    SELECT id, count(*) AS cnt
    FROM (
        SELECT id               -- grab ID of all teachers in database
        FROM bb_teacher
        UNION ALL --=--         -- lazily concatenate with
        SELECT id_teacher
        FROM bb_appointment     -- ID of teachers printed once for each hour of activity
    ) AS lambda
    GROUP BY id                 -- sort by ID and count occurence of each ID, giving grand total +1
    ORDER BY cnt ASC
) AS lambda
LIMIT 1                         -- END SELECTING LEAST ACTIVE TEACHER TODAY