Microsoft Office Tutorials and References
In Depth Information
Dependencies and Determinants
Tables are related to other tables. Fields are also related to other fields. Consider the mod-
ified Employee table shown in Figure A-11. Its description is:
Employee ( EmployeeNum , PositionID , LastName, PositionDesc, StartDate, HealthPlan,
A table combining fields from three tables
The modified Employee table combines several fields from the Employee, Position, and
Employment tables that appeared in Figure A-6. The EmployeeNum and LastName fields
are from the Employee table. The PositionID and PositionDesc fields are from the Position
table. The EmployeeNum, PositionID, and StartDate fields are from the Employment table.
The HealthPlan and PlanDesc fields are new fields for the Employee table, whose primary
key is now the combination of the EmployeeNum and PositionID fields.
In the Employee table, each field is related to other fields. To determine field relation-
ships, you ask “Does a value for a particular field give me a single value for another
field?” If the answer is Yes, then the two fields are related. For example, a value for the
EmployeeNum field determines a single value for the LastName field, and a value for the
LastName field depends on the value of the EmployeeNum field. In database discussions,
the word functionally is used, as in: “EmployeeNum functionally determines LastName”
and “LastName is functionally dependent on EmployeeNum.” In this case, EmployeeNum
is called a determinant. A determinant is a field, or a collection of fields, whose values
determine the values of another field. A field is functionally dependent on another field
(or a collection of fields) if that other field is a determinant for it.
You can graphically show a table’s functional dependencies and determinants in a
bubble diagram ; a bubble diagram is also called a data model diagram and a functional
dependency diagram . Figure A-12 shows the bubble diagram for the Employee table
shown in Figure A-11.