इसे आजमाएं ::
WHERE sd.student_break_date is null OR '2013-01-28' >= sd.student_break_date
या आप कोशिश कर सकते हैं ::
WHERE sd.student_break_date is null OR sd.student_break_date<'2013-01-28'
या ::
WHERE IFNULL(sd.student_break_date,'2013-01-27') <'2013-01-28'
