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 cannot use constraints in this case but you may create an updatable view with check option:

drop table t;
drop view v;
create table t (counter number);
create view v as select * from t where (select count(*) from t) <= 2 with check option;
insert into v values(1); -- OK
insert into v values(2); -- OK
insert into v values(3); -- OK
insert into v values(4); -- ERROR

You may adapt this example for your case.

An updatable view is view which can be treated as an ordinary table. You can create such a view with CHECK OPTION in this case Oracle will prevent you from executing DML on this view which don't relevant to the WHERE clause for this view.