Database Reference
In-Depth Information
INCORRECT DECOMPOSITIONS
It is important to note that the decomposition of a table into two or more third normal form tables must be
accomplished by the method indicated in the previous section, even though there are other possibilities that
at first glance might seem to be legitimate. For example, you can examine two other decompositions of the
Customer table into third normal form tables to understand the difficulties they pose. Assume in the decom-
position process that
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Customer (CustomerNum, CustomerName, Balance, CreditLimit, RepNum, LastName, FirstName)
is replaced by
Customer (CustomerNum, CustomerName, Balance, CreditLimit, RepNum)
Rep (CustomerNum, LastName, FirstName)
Samples of these tables appear in Figure 5-13. Both new tables are in third normal form. In addition, by
joining these two tables on CustomerNum, you can reconstruct the original Customer table. The result, how-
ever, still suffers from some of the same kinds of problems as the original Customer table.
