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This might very simple but I just had to check with you guys.

When it comes to databases, does the arrow in literature imply vise versa on equality?

Meaning, is A → B considered the SAME as B → A, in particular when it comes to databases and functional dependencies?

philipxy
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    Each username determines a single specific reputation score (at any given time). Does each reputation score determine a single specific username? – reaanb May 24 '17 at 11:32
  • You are not clear. "imply vise versa on equality" is unintelligible. And you don't mean "in particular when", you mean "when". – philipxy May 24 '17 at 20:15

1 Answers1

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Please read the reference(s) you were given for FDs (functional dependencies).

A FD is an expression of the form "A → B" for sets of attributes A & B. So if A and B are different, A → B is a different FD than B → A.

For a relation value or variable R, "A → B holds in R" and "A → B in R" say that if two R tuples have the same subtuple for A then they have the same subtuple for B.

Is A → B in R equivalent to B → A in R? If A and B are the same set, then yes. But what if they aren't?

X Y
a 1
b 1

{X} → {Y} holds in that relation value. {X} <> {Y}. Does {Y} → {X} also hold?

philipxy
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