Databases Reference
In-Depth Information
Second, there are currently situations where the same zip code corresponds to more than one city or even
to more than one state. This situation illustrates the wisdom in not making the change and the fact that
requirements and, consequently, the functional dependencies can change over time. It is critical to review
assumptions and dependencies periodically to see if any changes to the design are warranted.
Third, by splitting relations to achieve third normal form tables, you create the need to express an
interrelation constraint
, a condition that involves two or more relations. In the example given earlier for con-
verting to third normal form, you split the Customer relation in the Premiere Products database from
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Customer (CustomerNum, CustomerName, Balance, CreditLimit, RepNum, LastName, FirstName)
to
Customer (CustomerNum, CustomerName, Balance, CreditLimit, RepNum)
Rep (RepNum, LastName, FirstName)
Nothing about these two relations by themselves would force the RepNum on a row in the Customer rela-
tion to match a value of RepNum in the Rep relation. Requiring this to take place is an example of an inter-
relation constraint. Foreign keys handle this type of interrelation constraint. You will learn more about and
specify foreign keys during the database design process, which is covered in Chapter 6.
