Thanks alot of your instructions.i find my answer.

Thanks alot.

How about polymorphism in sql? Could we use stored procedures for polymorphism?

And for nested relationships can we define an attribute as OId and use it in a select query...

Thanks for your instruction, i am familiar with junction table, but i look a way to implement something like inheritance. In addition i agree with you for encapsulating by stored...

In inheritance a child table can gain father's features, so if i add child table's primary key to father's table as foreign key, it can gain father's features it is...