Database Reference
In-Depth Information
•
Each
Author
may write one or many
eBooks
.
•
Each
eBook
must be written by one
Author
.
Rather than repeat the relationship to
Author
, as well as the common attributes, we can
introduce subtyping:
•
Each
Author
may write one or many
Titles
.
•
Each
Title
must be written by one
Author
.
•
Each
Title
may be either a
PrintVersion
or
eBook
.
•
Each
PrintVersion
is a
Title
.
•
Each
eBook
is a
Title
.
The subtyping relationship implies that all of the relationships and attributes from the su-
pertype are inherited by each subtype. Therefore, there is an implied relationship from
Author
to
eBook
as well as from
Author
to
PrintVersion
. Also, the
titleName
, s
ub-
titleName
, and
titleRetailPrice
belong to
PrintVersion
and belong to
eBook
. Note that
each subtype must have the same attributes in their primary key as the supertype; in this
case,
titleISBN
, which is the identifier for a particular title.
