Oracle trigger that check constraint on a monthly basis

Question

just wonder is it possible to create a trigger to check on a specify constraint base on monthly basis.

eg.
table rent
|ID|Member|book|
-------------

Answer 1

If the table stores only actual data then it is quite simple:

create or replace trigger tr_rent
before insert on rent
for each row 
declare
  v_count number;
begin
  select count(*) into v_count
  from rent where member = :new.member;

  if v_count >= 4 then
    raise_application_error(-20001, 'Limit reached');
  end if;
end;
/

But if the table store historical data as well then you need some timestamp column, f.e. rent_date. So the count-query should be changed to the following:

select count(*) into v_count
from rent where member = :new.member
and rent_date > add_months(sysdate, -1);

In some cases reading a table that is currently being modified may lead to "mutating table error", but the trigger above is safe.