Database Reference
In-Depth Information
Advanced Data Model-
ing
Data Modeling
301
A fast-paced class covering techniques such as advanced normalization and
ragged hierarchies.
Tennis Basics
Tennis One
For those new to the game of tennis; learn the key aspects of the game.
Juggling
Learn how to keep three balls in the air at once!
Based on our definition of a candidate key (and a candidate key's characteristics of being
unique, stable, and minimal) what would you choose as the candidate keys for each of these
entities?
For
Student
,
Student Number
appears to be a valid candidate key. There are eight stu-
dents and eight distinct values for
Student Number
. So unlike
Student First Name
and
Student Last Name
, which can contain duplicates like
Eddie Murphy
,
Student Number
appears to be unique.
Student Birth Date
can also contain duplicates, such as
5/10/1982
,
which is the
Student Birth Date
for both
Mickey Mouse
and
Donald Duck
. However, the
combination of
Student First Name
,
Student Last Name
, and
Student Birth Date
may
make a valid candidate key.
For
Attendance
, we are currently missing a candidate key. Although the
Attendance Date
is unique in our sample data, we will probably need to know which student attended which
class on this particular date.
For
Class
, on first glance it appears that all of its attributes are unique and any of them
would therefore qualify as a candidate key. However,
Juggling
does not have a
Class Short
Name
. Therefore, because
Class Short Name
can be empty, we cannot consider it a can-
didate key. In addition, one of the characteristics of a candidate key is that it is stable.
I know, based on my teaching experience, that class descriptions can change. Therefore,
Class Description Text
also needs to be ruled out as a candidate key, leaving
Class Full
Name
as the best option for a candidate key.
Even though an entity may contain more than one candidate key, we can only select one
candidate key to be the primary key for an entity. A primary key is the candidate key that
has been chosen to be
the
unique identifier for an entity. An alternate key is a candidate key
that, although it has the properties of being unique, stable, and minimal, was not chosen as
the primary key but still can be used to find specific entity instances.
