An Introduction to Database Normalization - Preliminary Definitions (
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In
this section I introduce several definitions that are common jargon in the world
of database administration and normalization.
entity: The word
‘entity’ as it relates to databases can simply be defined as the general name
for the information that is to be stored within a single table. For example, if
I were interested in storing information about the school’s students, then
‘student’ would be the entity. The student entity would likely be composed of
several pieces of information, for example: student identification number, name,
and email address. These pieces of information are better known as
attributes.
primary key: A primary key uniquely identifies
a row of data found within a table. Referring to the school system, the student
identification number would be the primary key for the student table since an ID
would uniquely identify each student.
Note that a primary key might not
necessarily correspond to one specific attribute. In fact, it could be the
result of a combination of several components of the entity. For example, while
a location could not be a primary key for a class, since there might be several
classes held there throughout the day, the combined time and location would make
a satisfactory primary key, since no two classes could be held at the same time
in the same location. When multiple attributes are used to derive a primary key,
this key is known as a concatenated primary key.
relationship:
Understanding of the various relationships both between the data items forming
the various entities and between the entities themselves forms the crux of
database normalization. There are three types of data relationships that you
should be aware of:
one-to-one (1:1) - A one-to-one relationship signifies that each
instance of a given entity relates to exactly one instance of another entity.
For example, each student would have exactly one grade record, and each grade
record would be specific to one student.
one-to-many (1:M) - A one-to-many relationship signifies that each
instance of a given entity relates to one or more instances of another entity.
For example, one professor entity could be found teaching several classes, and
each class could in turn be mapped to one professor.
many-to-many (M:N) - A many-to-many relationship signifies that many
instances of a given entity relate to many instances of another entity. To
illustrate, a schedule could be comprised of many classes, and a class could be
found within many schedules.
foreign key: A foreign key forms the
basis of a 1:M relationship between two tables. The foreign key can be found
within the M table, and maps to the primary key found in the 1 table. To
illustrate, the primary key in the professor table (probably a unique
identification number) would be introduced as the foreign key within the classes
entity, since it would be necessary to map a particular professor to several
classes.
Entity-relationship diagram (ERD): An ERD is essentially
a graphical representation of the database structure. These diagrams, regardless
of whether they are built using the latest design software or scrawled on a
napkin with a crayon, are immensely useful towards attaining a better
understanding of the dynamics of the various database relationships. Click
here to examine a sample ERD diagram which illustrates the relational
structure that might be found in our school system
database.
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