Database Reference
In-Depth Information
tool for thinking about data structures. Given any tuple T, with two sets of attributes
{X
1……….
...X
n
} and {Y
1
............Y
n
} , then set Y is functionally dependent on set X if, for
any legal value of X, there is only one legal value for Y.
The functional dependency between sets of attributes can be indicated as shown in figure.
Functional dependencies can be expressed as X (ProductID)Y(Description), which reads
“X functionally determines Y.”
Example 1:
ProductID Description
Example 2:
In a college personnel database, Staff_No and Staff_Name are in a functional
determinant relationship. Staff_No is the determinant and Staff_Name is the dependent
data-item. For every Staff_No there is only one associated value of Staff_Name.
For example, 135 may be associated with the value G. P. Singh. This does not mean that
there are not more than one member of staff named G. P. Singh in a college. It simply
means that each G. P. Singh will have a different staffNo. There is a functional determ-
inancy from Staff_No to Staff_Name but the same is not true in the opposite direction.
Staff_No will probably functionally determine Department_Name. For every member of
staff there is only one associated department that applies.
