MySQL timestampdiff interval

DELETE FROM 
# SECOND  second 
# MINUTE  minute （ Returns the difference in seconds divided by 60 The integral part of ）
# HOUR  Hours （ Returns the difference in seconds divided by 3600 The integral part of ）
# DAY  Days （ Returns the difference in seconds divided by 3600*24 The integral part of ）
# MONTH  Monthly number 
# YEAR  number of years 
# year ,month ,day,hour,second, minute ,start_time ,submit_time
exam_record WHERE score<60 and TIMESTAMPDIFF(minute,start_time,submit_time)<5

DELETE
FROM exam_record WHERE submit_time is null or TIMESTAMPDIFF(minute,start_time,submit_time)<5 ORDER BY start_time LIMIT 3