hibernate中声明集合属性一般都需要初始化，因为要像其中添加数据。private Set<Student> students; 这行代码如果想调试时不提示空指针
可以初始化一下：private Set<Student> students = new HashSet<Student>();
额，执行SQL语句报错~
[Err] 1055 - Expression #1 of ORDER BY clause is not in GROUP BY clause and contains nonaggregated column 'information_schema.PROFILING.SEQ' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
