Database Reference
In-Depth Information
same type of data. In this chapter, if a table is qualiied as a relation, we may use the word
relation
in the process of improving the table structure. he word
relationship
refers to the association
between tables.
3.2.1 Functional Dependency
In a table, the data values in some columns determine the data values in other columns. For
example, in Figure 3.1, each value in the column LocationID can determine a value in the column
CostRate. On the other hand, a single value in CostRate cannot determine a value in the column
LocationID. hat is, the value 0 in the column CostRate corresponds to several diferent values in
the column LocationID. On the basis of the above discussion, we say that LocationID determines
CostRate but not the other way around. LocationID is called a
determinant
, and CostRate that is
determined by LocationID is called a
dependent
. he notation used to represent the dependency
is shown below:
LocationID
→
CostRate
he above notation means that CostRate is
functionally dependent
on LocationID. By
examining Figure 3.1, there is another functional dependency.
LocationID
→
Name, CostRate, Availability, ModiiedDate
his indicates that Name, CostRate, Availability, and ModiiedDate are
functionally depen-
dent
on LocationID.
In a data model,
functional dependency
is a relationship among attributes where one attri-
bute or a combination of attributes determines another attribute. he concept of functional depen-
dency can be deined more formally in the following theorem:
If an attribute X determines another attribute Y, the attribute Y is
functionally dependent
on the attribute X.
Sometimes, a determinant is formed by a combination of attributes. As shown in Figure 3.3,
lname or fname alone is not a determinant for other columns since the repeated value Anderson
in lname cannot be used to determine the values Ben and Donna in fname and the repeated
value Mary in fname cannot be used to determine the values Collins and Garza in lname. If you
Figure 3.3
Combination determinant.
