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

You can't use check constraints on subqueries for this. You can, however, use triggers. A simple use of row-level trigger on a table selecting from the same table and raising an exception on count > 4 would result in ORA-4901, so you'd need composite triggers for this. But that is a bit of overkill already.

So, I suggest that you might use a separate table with aggregated counts of user_ids updated via trigger and check-constrain that count. Source code:

create table user_aggr
(
    user_id             integer not null primary key,
    user_count          integer not null check (user_count <= 4)
);

You'll need to create a foreign key reference to the user_aggr table on your user table (named user_registration in my example):

create table user_registration
(
    registration_id             integer primary key,
    user_id                     integer references user_aggr
);

... or just creating the FK constraint ...

alter table user_registration
add constraint FK_user_registration (user_id)
references user_aggr;

... plus the necessary index over the FK ...

create index user_registration_i0
on user_registration (user_id);

Then you'll need the calculation trigger:

create or replace trigger auto_user_counter
    after insert or delete or update of user_id
    on user_registration
    for each row
begin
    if updating or deleting then
        merge into user_aggr T
        using dual
        on ( T.user_id = :old.user_id )
        when matched then
            update set T.user_count = T.user_count - 1
            delete where T.user_count = 0
        ;
    end if;

    if inserting or updating then
        merge into user_aggr T
        using dual
        on ( T.user_id = :new.user_id )
        when matched then
            update set T.user_count = T.user_count + 1
        when not matched then
            insert (user_id, user_count) values (:new.user_id, 1)
        ;
    end if;
end;
/

And now you're all set and ready to rumble. Examples ...

insert into user_registration (registration_id, user_id) values(1, 1); -- OK
insert into user_registration (registration_id, user_id) values(2, 1); -- OK
insert into user_registration (registration_id, user_id) values(3, 1); -- OK
insert into user_registration (registration_id, user_id) values(4, 1); -- OK
insert into user_registration (registration_id, user_id) values(5, 1); -- ERROR

insert into user_registration (registration_id, user_id) values(11, 2); -- OK
insert into user_registration (registration_id, user_id) values(12, 2); -- OK
insert into user_registration (registration_id, user_id) values(13, 2); -- OK
insert into user_registration (registration_id, user_id) values(14, 2); -- OK
insert into user_registration (registration_id, user_id) values(15, 2); -- ERROR

delete from user_registration where user_id = 2 and rownum <= 1; -- OK; user_id 1: 4 rows; user_id 2: 3 rows
update user_registration set user_id = 2 where user_id = 1 and rownum <= 1; -- OK; user_id 1: 3 rows; user_id 2: 4 rows
update user_registration set user_id = 2 where user_id = 1 and rownum <= 1; -- ERROR on user_id 4