>> Partial Grid View of Person Instances:
>> ID name phone# email staffMember
>> -- --------- -------- ------------- -----------
>> # judy 333-5555 j...@[EMAIL PROTECTED]
clark
>> # clark 737-5588
>>
>> Partial Tree View of Person Instances:
>> -Person
>> |-instance-
>> |-Judy
>> | |-phone#-
>> | | |-333-5555
>> | | |-JDG-JUDY
>> | |
>> | |-email-
>> | | |-J...@[EMAIL PROTECTED]
>> | | |-J...@[EMAIL PROTECTED]
>> | |
>> | |-staffMember-
>> | |-Clark+
>> | |-Ashley+
>> | |-Colby+
>> | |-Brandy+
>> | |-Courtney+
>> |
>> |-Clark
>> | |-phone#-
>> | | |-737-5588
>> | |
>> | |-assistant-
>> | |-Ashley+
>
> This is a typical structure of a "navigational" or "hierarchical"
database. They are fine for particular specific uses, but over time people
found they grew into messes and are hard to query for unforseen questiones.
Dr. Codd was working with stuff like this when he felt there should be a
better way.
Dr. Codd was right. The Relation Data Model is more general/flexible
than the Hierarchal Data Model. The advantage of a more
general/flexible method (RM) is that its scope it larger. The advantage
of a more specialized/limited method (HM) is that it is more efficient
within its smaller scope. (This might sound familiar as I have been
repeating this for several years now :)
However, to assume the underlying data model based on a particular view
is unreliable. In hierarchal model, it is impossible to represent a
thing with more than one parent without redundancy. If you were to
click around in exp db's tree view, with option to display each thing's
ID enabled, you would see that while a thing is a child in multiple
hierarchies, it still has the same ID. For example, most of the persons
in the example have multiple classifications, yet when one drills down
Person, StaffMember or Clerk instances, Clark has the same ID! The same
can be verified with this thread's original example where Adam is in
the parent/child and boss/employee hierarchy. If you change Clark's or
Adam's attributes or atribute values in one hierarchy, it shows up in
the other hierarchies as well, without the db engine having
synchronized data in multiple places. (Note that tree nodes in GUI may
need to be refreshed by closing/opening its parent tree node)
|
