CREATE OR REPLACE TRIGGER trg_schedule
BEFORE INSERT OR UPDATE ON schedule
FOR EACH ROW
DECLARE
  flag NUMBER;
BEGIN
  select case 
    when exists
      (select null from teachers_subjects where teacher_id=:new.teacher_id and subject_id=:new.subject_id)
    then 1
    else 0
    end into flag from dual;
  if (flag=0) then
    raise_application_error('-20000','Teacher with id='||:new.teacher_id||' doesn''t teach the subject with id='||:new.subject_id);
  end if;
END;
/

--drop table teacher cascade constraints purge;
--drop table subject cascade constraints purge;
--drop table teachers_subjects cascade constraints purge;
--drop table schedule cascade constraints purge;

create table teacher(id number primary key, fullname varchar2(50) not null);
insert into teacher(id, fullname) values(1, 'Marivanna');
insert into teacher(id, fullname) values(2, 'Sansanych');
create table subject(id number primary key, discipline_name varchar2(30) not null);
insert into subject(id, discipline_name) values(1, 'Math');
insert into subject(id, discipline_name) values(2, 'Geography');
insert into subject(id, discipline_name) values(3, 'Chemistry');
create table teachers_subjects (id number primary key, teacher_id number references teacher(id) not null, subject_id number references subject(id) not null);
insert into teachers_subjects (id, teacher_id, subject_id) values (1,1,1);
insert into teachers_subjects (id, teacher_id, subject_id) values (2,2,2);
create table schedule(id number primary key, teacher_id number references teacher(id) not null, subject_id number references subject(id) not null, begin_datetime date not null, end_datetime date not null);

CREATE OR REPLACE TRIGGER trg_schedule
BEFORE INSERT OR UPDATE ON schedule
FOR EACH ROW
DECLARE
  flag NUMBER;
BEGIN
  select case 
    when exists
      (select null from teachers_subjects where teacher_id=:new.teacher_id and subject_id=:new.subject_id)
    then 1
    else 0
    end into flag from dual;
  if (flag=0) then
    raise_application_error('-20000','Teacher with id='||:new.teacher_id||' doesn''t teach the subject with id='||:new.subject_id);
  end if;
END;
/

delete from schedule;

insert into schedule (id, teacher_id, subject_id, begin_datetime, end_datetime) values (1,1,1,sysdate,sysdate+5/1440);
insert into schedule (id, teacher_id, subject_id, begin_datetime, end_datetime) values (2,2,2,sysdate,sysdate+5/1440);
insert into schedule (id, teacher_id, subject_id, begin_datetime, end_datetime) values (3,1,2,sysdate,sysdate+5/1440);
update schedule set subject_id = 3 where teacher_id=1;