分组：
select * from student2 where student_sex='男' order by student_age desc,student_id asc
select * from student2 where student_sex='男' order by student_id asc
select * from student2 where student_sex='男' order by student_age desc
select * from student2 where student_sex='男' order by student_id asc
select * from student2 where student_sex='女' order by student_name desc
排序：
select COUNT(*) from student2
select COUNT(*) from student2 where student_sex='男'
select COUNT(*) from student2 where student_sex='女'
select COUNT(*) from student2 group by student_sex
