Syntax:
YEARWEEK(date), YEARWEEK(date,mode)

Returns year and week for a date. The mode argument works exactly like
the mode argument to WEEK(). The year in the result may be different
from the year in the date argument for the first and the last week of
the year.

URL: http://dev.mysql.com/doc/refman/5.6/en/date-and-time-functions.html

