WEEK(date[,mode])

此函数返回日期的周数。双参数的形式WEEK()允许你指定星期是否开始于周日或周一，以及是否返回值应在范围从0到53或从1到53。 如果省略了mode参数，系统default_week_format变量的值被使用

Mode	一个星期的第一天	范围	Week 1 是第一个星期
0	Sunday	0-53	with a Sunday in this year
1	Monday	0-53	with more than 3 days this year
2	Sunday	1-53	with a Sunday in this year
3	Monday	1-53	with more than 3 days this year
4	Sunday	0-53	with more than 3 days this year
5	Monday	0-53	with a Monday in this year
6	Sunday	1-53	with more than 3 days this year
7	Monday	1-53	with a Monday in this year

mysql> SELECT WEEK('1998-02-20');
+---------------------------------------------------------+
| WEEK('1998-02-20')                                      |
+---------------------------------------------------------+
| 7                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)