sql frist day

计算周月的第一天, 以星期天为一周的开始

	SELECT LAST_DAY('2013-2-1');
	SELECT concat(date_format(LAST_DAY('2013-02-01'),'%Y-%m-'),'01');
	select DATE_SUB('2014-03-08',INTERVAL (WEEKDAY('2014-03-08')+1)%7 DAY);
	select DATE_SUB('2014-03-08',INTERVAL ((WEEKDAY('2014-03-08')+1)%7)-6 DAY);