Entity Hierarchies
--------------------


Type hierarchy: Any subclass wil inherit the attributes of the
super class

Instance Hierarchy: All capitals are in Cities. Objects in the
subclasses also belong in the parent hierarchy.

Movies union TVShowEpisodes = ? = AllMedia

No...: the hierarchy is covering


Movies intersection TVShowEpisodes = emptyset

Yes: the hierarchy is disjoint

Converting Entity Hierarchies to Relational Data Model
=========================================================

Given a parent entity and subclass entities:

Option 1:
-----------
Convert each subclass entity to a separate relation (if the hierarchy
is covering, then the parent entity does not need a separate relation)

Cities(id, name, population)
Capitals(id, name, population, country)

Movies(id, mediafile, title)
TVShowEpisodes(id, mediafile, title)

Pictures(id, image, movieid, tvshowepisodeid)

PicturesM(id, image, movieid)
PicturesTV(id, image, tvshowepisodeid)

Students(RIN, name, address, email, class)
Faculty(RIN, name, address, email, title, rank)
Staff(RIN, name, address, email, title)

Pros?
Precise!

Cons?

Multiple entities copy the same information

-------------

Option 2:
---------
Convert the parent entity to its relation and then create a new
relation for each subclass, only listing the unique attributes

Cities(id, name, population)
Capitals(id, country)

Media(id, mediafile, title)
Movies(id)
TVShowEpisodes(id, seasonNo, showName)
Pictures(id, image, mediaid)

People(RIN, name, address, email)
Students(RIN, class)
Faculty(RIN, title, rank)
Staff(RIN, title)



Pros?
No repeated tuples

Cons?
Need a lot of joins to get the same info


Option 3:
----------
Flatten and create a single entity for the whole hierarchy. We need an
attribute(s) to identify the class(es) the entity belongs to.

Cities(id, name, population, country, isCapital?)

Media(id, title, mediafile, mediaType(isMovie), seasonNo, showName)

disjoint

People(RIN, name, address, email, class, title, rank, isStudent, isFaculty, isStaff)

not disjoint

Pros?
No additional joins

Cons?
Waste space with lots of empty attributes
Not elegant: not modular

---------

Design Principles:

- Merge entities that are very similar into a single one

- Break apart entities that are different (different relationships or
  attributes)

- If information is present from other part of the ER diagram, do not
  add them

- Only represent the information that is necessary