Database Reference
In-Depth Information
We learned that although
Phone Number 1
,
Phone Number 2
, and
Phone Number 3
ap-
pear as repeating attributes, they are really three different pieces of information based upon
the sample values we were given.
Phone Number 3
contained the same value for all four
employees, and after validating with the business expert we learned that this is the organ-
ization's phone number.
Phone Number 2
varied by department, so this attribute was re-
named to
Department Phone Number. Phone Number 1
is different for each employee,
and we learned that this is the
Employee Phone Number
. We also were told that
Employ-
ee Name
does contain more than one piece of information, and therefore it should be split
into
Employee First Name
and
Employee Last Name
.
Second Normal Form (2NF)
Recall that the series of rules can be summarized as:
Every attribute is single-valued and
provides a fact completely and only about its primary key
. First Normal Form (1NF) is
the “single-valued” part. Second Normal Form (2NF) is the “completely” part. This means
each entity must have the minimal set of attributes that uniquely identifies each entity in-
stance.
As with 1NF, we will find ourselves asking many questions to determine if we have the
minimal primary key. We can have another question template such as: “Are all of the at-
tributes in the primary key needed to retrieve a single instance of [[insert attribute name
here]]?” In our Employee data model, the minimal set of primary key instances are
Em-
ployee Identifier
and
Department Code.
So these are all valid questions for our employee
example:
