一个老师教多个学生，一个学生有多个老师。
class Teacher {
String id;
String name;
Set students;
}
class Student {
String id;
String name;
Set teachers;
}
现在要查询教某一个学生（name为xxx）的老师。
HSQL方法一：（性能最优）
select elements(s.teachers) from Student s where s.name = 'xxx';
HSQL方法二：（要用到 inner join）
select t from Teacher t join t.students s where s.name = 'xxx';
HSQL方法三：（要用到子查询）
select t from Teacher t, Student s where s in elements(t.students) and s.name = 'xxx';
