Mysql: Problem using GREATEST with IFNULL and join
SELECT
    GREATEST
    (
        IFNULL(tableb.create_time,FROM_UNIXTIME(18000)),
        IFNULL(tablea.create_time,FROM_UNIXTIME(18000))
    );


It just so happens that FROM_UNIXTIME(18000) is '1970-01-01 00:00:00'