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why can't we say that A functionally determines B from the given relation instance when it satisfies the constraints since for unique value of A we have unique value of B this implies that A functionally determines B , so then why can't we conclude whether A -->B holds or not ?

2 Answers

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1 votes
A functionally determines B , we can`t say by looking at the instances . Later when the table is updated A -> B this may not hold .

So, for one instances this may hold ( A->B) but another instances this may not hold .
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A->B hold on this relation instance, but unless the complete relation schema is known, we cannot say for sure if A->B holds on the entire schema. It may be the case that for some other tuples. this dependency is not satisfied. One thing that is sure is B does not functionally determine C , whether you consider this instance or the entire schema

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