hibernate中聲明集合屬性一般都需要初始化，因為要像其中添加數據。private Set<Student> students; 這行代碼如果想調試時不提示空指針
可以初始化一下：private Set<Student> students = new HashSet<Student>();
額，執行SQL語句報錯~
[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
