DISTINCT /EXTRACT

SELECT DISTINCT(EXTRACT(MONTH FROM start_date_time)) 
  FROM examination

DISTINCT:去除重复的项

   select distinct name, id from table ：//去除name和id都相同的项

EXTRACT：取出时间对应的项、

   mysql:参考 http://www.java2s.com/Tutorial/MySQL/0280__Date-Time-Functions/SELECTEXTRACTYEARMONTHFROM19990702010203.htm