WEEKDAY(date)
返回日期的星期索引(0=周一，1=周二，6=星期日)。
mysql> SELECT WEEKDAY('1998-02-03 22:23:00');
+---------------------------------------------------------+
| WEEKDAY('1998-02-03 22:23:00') |
+---------------------------------------------------------+
| 1 |
+---------------------------------------------------------+
1 row in set (0.00 sec)
