<database> Given a relation R (in a relational database), attribute Y of
R is functionally dependent on attribute X of R and X of R functionally
determines Y of R (in symbols R.X -> R.Y) if and only if each X in R has
associated with it precisely one Y in R (at any one time). Attributes X and Y
may be composite.
This is very close to a function in the mathematical sense.
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