我有3张桌子
classes
id class
1 10th
2 9th
3 8th
....
students
id name class_id...
1 smith 2
2 gyle 3
......
attendances
id date student_id attendance
1 2014-01-01 1 P
2 2014-01-01 2 A
3 2014-01-02 1 P
4 2014-01-02 2 P
.....
我想在每个（分组）日期显示每个班级的现在学生总数如下
date 10th 9th 8th .....
2014-01-01 12 11 13 .....
2014-01-02 13 11 12 .....
2014-01-01 13 10 13 .....
....................................