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

Returns year and week for a date. 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.

The mode argument works exactly like the mode argument to WEEK(). For
the single-argument syntax, a mode value of 0 is used. Unlike WEEK(),
the value of default_week_format does not influence YEARWEEK().

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

