How to apply check constraint at a row level?

Question

I am new to SQL. I have a schema like this. Students(student_id,student_name,club,date_of_passed_out). clubs(club_name,club_in_charge,club_inaugurated_date). The students can act as the club in charges. So it is not possible for a student to act as a club in-charge if his date_of_passed_out is less than club_inaugurated_date. I must prevent accidental inputs where the club inauguration date is greater than the date of pass out of the club in-charge. How can I achieve this. Thanks in advance.

Answer 1

Hi,
There should be three tables for your requirement.
Table1: Clubs ( club_name, club_inauguration_date)  -- club_name is Primary Key
Table2: Students(student_id,student_name,club_name,date_of_passed_out) -- student_id primary key, club_name - Foreign Key
Table3: club_inchrg_detail ( club_name,club_incharge); -- club_incharge is - students(student_id)
SQL> create or replace trigger club_inchrg_trig
  2  after insert or update on clubs
  3  for each row
  4  declare
  5      l_passed_out  date;
  6  begin
  7
  8        select date_of_passed_out
  9          into l_passed_out
10        from students
11        where student_id = :new.club_in_charge;
12
13    if inserting or updating then
14        if :new.club_inaugurated_date <= l_passed_out then
15              raise_application_error(-20100,' Student is not eligible for club incharge ');
16        end if;
17    end if;
18  end;
19  /
Trigger created.
As I already mentioned, you should have three tables for this requirement, and also I didnt handle the exceptions. This code is just for demonstration purpose only.