SQL - Trigger to check insert of date on each student - sql

Question

I'm trying to create a trigger that check the date that I'm inserting is greater than the one present in table for each student. The control need to be only on ID and DATE.
STUDENT_EXAMS
id_student subject mark date_exam
1 Chemistry 6 'May-05-2020'
2 Maths 7 'May-01-2020'
LEGITIMATE INSERT
insert into STUDENT_EXAMS (id_student, subject, mark, date_exam)
values (1, 'History', 8, 'May-06-2020');
insert into STUDENT_EXAMS (id_student, subject, mark, date_exam)
values (2, 'Biology', 8, 'May-05-2020');
ILLEGITIMATE INSERT
insert into STUDENT_EXAMS (id_student, subject, mark, date_exam)
values (1, 'History', 8, 'May-04-2020');
insert into STUDENT_EXAMS (id_student, subject, mark, date_exam)
values (2, 'Biology', 10, 'Apr-30-2020');
This is the trigger that I tried to create, but it's not working and I don't know how to insert also a control on each ID_STUDENT.
create or replace trigger check_date
before insert on STUDENT_EXAM
for each row
begin
if (:new.date_exam > :old.date_exam) then
insert into STUDENT_EXAM (id_student, subject, mark, date_exam)
values (:new.id_student, :new.subject, :new.mark, :new.date_exam);
end if;
end;

Answer 1

Perhaps this is the one that you are interested in.It prevents insertion to STUDENT_EXAM table when date_Exam for row being inserted is less than max value of date_Exam for the id.
create or replace trigger check_date
before insert on STUDENT_EXAM
for each row
DECLARE
lv_date_exam DATE;
begin
select max(date_exam) into lv_date_exam
from student_exam where id = :new.id;
if (:new.date_exam < lv_date_exam) then
raise_application_error(-20000
, 'Cannot insert record as date_exam '||:new.date_exam||' is less than max date_exam '||lv_date_exam);
end if;
end;
[EDITED by Littlefoot, to show why it will fail with the mutating table error]
Table & trigger you suggested:
SQL> create table student_exam (id number, date_exam date);
Table created.
SQL> create or replace trigger check_date
2 before insert on STUDENT_EXAM
3 for each row
4 DECLARE
5
6 lv_date_exam DATE;
7 begin
8
9 select max(date_exam) into lv_date_exam
10 from student_exam where id = :new.id;
11
12 if (:new.date_exam < lv_date_exam) then
13 raise_application_error(-20000
14 , 'Cannot insert record as date_exam '||:new.date_exam||' is less than max date_exam '||lv_date_exam);
15 end if;
16 end;
17 /
Trigger created.
Testing:
This works:
SQL> insert into student_exam (id, date_exam) values (1, sysdate);
1 row created.
But this does not:
SQL> insert into student_exam (id, date_exam)
2 select 1, sysdate - 10 from dual union all
3 select 1, sysdate + 20 from dual;
insert into student_exam (id, date_exam)
*
ERROR at line 1:
ORA-04091: table SCOTT.STUDENT_EXAM is mutating, trigger/function may not see
it
ORA-06512: at "SCOTT.CHECK_DATE", line 6
ORA-04088: error during execution of trigger 'SCOTT.CHECK_DATE'
SQL>