l'uso di un trigger, sebbene sembri funzionare, è un modo pericoloso per farlo, poiché le sessioni in esecuzione in parallelo non vedranno i dati inseriti dalla sessione in corso. Puoi farlo tramite una vista materializzata:
SQL> create table rent (id number primary key, member varchar2(30), book varchar2(20), date_rented date);
Table created.
SQL> create index rent_ix1 on rent ( member, date_rented);
Index created.
SQL> create materialized view log on rent with rowid(member,date_rented)
2 including new values;
Materialized view log created.
SQL> create materialized view rent_month_check
2 refresh fast on commit
3 as
4 select trunc(date_rented, 'mm') month, member, count(*) rentals
5 from rent
6 group by trunc(date_rented, 'mm'), member;
Materialized view created.
SQL> alter table rent_month_check
2 add constraint rent_month_check_ck1 check (rentals <= 4);
Table altered.
SQL> insert into rent values(1, 'DazzaL', 'crime', sysdate);
1 row created.
SQL> commit;
Commit complete.
SQL> insert into rent values(2, 'DazzaL', 'mystery', sysdate+1);
1 row created.
SQL> commit;
Commit complete.
SQL> insert into rent values(3, 'DazzaL', 'fantasy', sysdate+2);
1 row created.
SQL> commit;
Commit complete.
SQL> insert into rent values(4, 'DazzaL', 'politics', sysdate+3);
1 row created.
SQL> commit;
Commit complete.
SQL> insert into rent values(5, 'DazzaL', 'thriller', sysdate+4);
1 row created.
SQL> commit;
commit
*
ERROR at line 1:
ORA-12008: error in materialized view refresh path
ORA-02290: check constraint (TEST.RENT_MONTH_CHECK_CK1) violated
SQL> select * from rent_month_check;
MONTH MEMBER RENTALS
--------- ------------------------------ ----------
01-NOV-12 DazzaL 4
