Database Reference
In-Depth Information
4.
If we consider S suppliers, each supplying N items to J
projects, then it does not take much imagination to see that a
fair amount of duplication will take place, despite the fact that
the relation is in 4NF.
Let us examine a possible decomposition of
SupplierSchedule
as shown in Figure
4-5
.
If we employ the first two decompositions only, this will not result in a situation that
will guarantee us the original
SupplierSchedule
. In fact, if we were to join these two
decompositions (
SI
and
IP
), we would obtain a false representation of the original
relation. The third projection (
PS
) is absolutely necessary, if we are to have any guarantee
of obtaining the original relation after joining the projections.
Figure 4-5.
Illustrating Possible Decompositions of Supplier-Schedule
