Lecture 8:

Revisiting Entities:
-----------------------

Multi-valued attributes
Weak entities


Example:
------------

Albums have:
    multiple genres
    multiple styles
    multiple userratings
    multiple similar albums
    multiple releases (each with year/catalog/format)
    multiple tracks (each track, 1,2,3,... features one song)



Weak Entities
-----------------------------
Drawn with a box with two lines around it):

   A weak entity has a key and a set of attributes. 

   The key of a weak entity is not guaranteed to be unique in the
   database.

   A weak entity depends on one of more entities with a many-to-one
   relationships called supporting entities. The key of all supporting
   entities combined with the key of the weak entity is guaranteed to
   be unique in the database. The supporting entities connect to the
   weak entity by supporting relationships, drawn with two lines around them.

   Each weak entity and all supporting relationships together are
   mapped to a new relation. The key of the relation is the
   combination of the weak entity key and the key of all the
   supporting entities.


Track as weak entity (model 2):

Track(AlbumId, TrackNo, duration, notes, songid)  Key: AlbumId, TrackNo

Track as strong entity (model 1):

Track(Id, TrackNo, duration, notes, songid, albumid)  Key: Id