--建表语句
create table student(id number(9),name varchar2(64),age number(5));create table class(id number(9),name varchar2(64),teacherid number(9),studentid number(9));create table teacher(id number(9),name varchar2(64),age number(5)); 
--学生信息
insert into student values(1,‘yangml‘,23);
insert into student values(2,‘zhangj‘,23);
insert into student values(3,‘zhulj‘,26);
insert into student values(4,‘chenglp‘,26);
--老师信息
insert into teacher values(1,‘yangls‘,33);
insert into teacher values(2,‘zhangls‘,33);
insert into teacher values(3,‘zhuls‘,33);
insert into teacher values(4,‘chenls‘,33);
--班级信息
insert into class values(1,‘一年级‘,1,1);
insert into class values(2,‘二年级‘,2,2);
insert into class values(3,‘三年级‘,3,3);
insert into class values(4,‘四年级‘,4,4);

--删除触发器
create or replace trigger trigdel
  before delete on class  
  for each row
declare
  -- local variables here
begin
   delete from teacher where teacher.id=:old.id; 
   delete from student where student.id=:old.id;
end trigdel;
--测试删除
delete from class;
select * from class;
select * from student;
select * from teacher;