How to use oracle check constraints to limit number of registration?

Question

I\'ve a user table with unique user_id. User can register using there id. Now I want to limit the max. registration per user using CHECK constraints. so I use this:

Answer 1

Under certain conditions, you can enforce table restrictsion with materialized views:

create table tq84_t (
  user_id   number,
  foo       varchar2(10),
  constraint pk_tq84_t primary key (user_id, foo)
);

create materialized view log on tq84_t;

create materialized view tq84_mv 
 refresh on commit
as
  select user_id, count(*) cnt
    from tq84_t
   group by user_id;

alter table tq84_mv
  add constraint check_max_2_registrations 
  check (cnt < 3);

With this materialized view, Oracle checks the constraint on the materialized view when you commit:

insert into tq84_t values (1, 'a');
insert into tq84_t values (1, 'b');

commit;

This works. The following doesn't:

insert into tq84_t values (1, 'c');

commit;

It fails with

ORA-12008: error in materialized view refresh path
ORA-02290: check constraint (META.CHECK_MAX_2_REGISTRATIONS) violated