Some really confusing things about parent-child relationship

I would really need some help with the following questions. And apologies for so many of them

1)

a)
In hierarchical model parent class can have several subclasses. What does that mean exactly?

That an instance A of parent class can have connections to several instances of certain subclass, or that an instance A can have connections to two or more instances, where each instance is of different subclass type, or both?

b)
And why do we name some classes subclasses and other classes parents?
It makes sense in object oriented programs, but here subclass doesn't necessarily suggests more specialized version of parent class. As far as I understand it, in hierarchical model we get a parent child relationship by simply creating a connection between two classes?!

2)
My book only briefly talks about network model, so some of the information on the subject is very ambiguous. For example:

a)
with network model child classes can have several parent classes --> do we call this a M:N connection, or does M:N connection only mean that an instance of certain child class is connected to several instances of certain parent class and vice versa?

Anyhow, book divides network model in three parts:
* simple network model --> M:N connections are not allowed

• complex network model --> M:N connections are allowed
• limited network model

b)
Are in all three cases situations allowed where a child can have several parents?

c)
 Now how do we know which of the two connected entity types is a parent and which a child and why does it matter?

d)
Next comes the mother of all confusing statements:  book starts claiming that between the two entity types there can be several 1:N connection ( meaning just 1:N connections ... M:N connections ).
Huh, didn't it previously claimed that complex network model allows M:N connections?

thank you Received on Tue Aug 14 2007 - 20:15:04 CEST