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Consider the following instance r(R) of the relation schema R(ABCDE):
Figure 11.1
What kind of dependencies can we observe among the attributes in Table R?
- Since the values of A are unique, it follows from the FD definition that:
- A →B, A →C, A →D, A →E
- It also follows that A →BC (or any other subset of ABCDE).
- This can be summarized as A →BCDE
- From our understanding of primary keys, A is a Primary Key.
- Since the values of E are always the same, it follows that: A →E, B →E, C →E, D →E However, we cannot generally summarized above by ABCD →E In general, A →E, B →E AB →E
Other observations:
combinations of BC are unique, therefore BC →ADE
combinations of BD are unique, therefore BD →ACE
if C values match, so do D values, therefore C →D however, D values don’t determine C values, so C does not determine D, and D does not determine C.
When looking at the data, it makes a lot more sense in terms of which attributes are dependent and which are determinants.
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