Databases Reference
In-Depth Information
and, after all, a person can have only one last name. Informally, we might say
that Salesperson Number defines Salesperson Name. If I give you a Salesper-
son Number, you can give me back the one and only name that goes with it.
These defining associations are commonly written with a right-pointing arrow
like this:
Salesperson Name
In the more formal terms of functional dependencies, the attribute on the
left side is referred to as the
determinant attribute.
This is because its value
determines the value of the attribute on the right side. Conversely, we also
say that the attribute on the right is functionally dependent on the attribute
on the left.
Data normalization is best explained with an example. In order to demon-
strate the main points of the data normalization process, we will modify part
of the General Hardware Company business environment and focus on the
SALESPERSON and PRODUCT entities. Let's assume that salespersons are
organized into departments and that each department has a manager who is
not herself a salesperson. Then the list of attributes that we will consider is
shown in Figure 4-23.
The list of defining associations or functional dependencies is shown in
Figure 4-24. Notice a couple of fine points about the list of defining associations
in Figure 4-24. The last association:
Salesperson Number
→
Quantity
shows that the combination of two or more attributes may define another
attribute. That is, the combination of a particular Salesperson Number and a par-
Salesperson Number, Product Number
→
Figure 4-23
Salesperson Number
Salesperson Name
Commission
Percentage
Year of Hire
Department
Number
Manager Name
Product Number
Product Name
Unit Price
Quantity
Salesperson entity attributes.
