Allow only 3 rows to be added to a table for a specific value

Question

I have a question in hand where i need to restrict the number of projects assigned to a manager to only 3. The tables are:

Manager:
Manager_employee_id(PK)
M         


        

Answer 1

"How do I implement the restrict to 0,3?"

This requires an assertion, which is defined in the SQL standard but not implemented in Oracle. (Although there are moves to have them introduced).

What you can do is use a materialized view to enforce it transparently.

create materialized view project_manager
refresh on commit 
as 
select Project_manager_employee_id
        , count(*) as no_of_projects
from project
group by Project_manager_employee_id
/

The magic is:

alter table project_manager
   add constraint project_manager_limit_ck check 
       ( no_of_projects <= 3 )
/

This check constraint will prevent the materialized view being refreshed if the count of projects for a manager exceeds three, which failure will cause the triggering insert or update to fail. Admittedly it's not elegant.

Because the mview is refreshed on commit (i.e. transactionally) you will need to build a log on project table:

create materialized view log on project