A succinct definition of a functional dependency, such as X->Y, is that whenever X assumes a certain value (say x), and for that value of X (x), Y assumes a certain value y, then in every subsequent retrievals of X, whenever X assumes the value x, Y should assume the value y. Thats it! Functional dependency is no more than that. It is a very simple mathematical idea. It is the job of the subsequent normal forms to reduce the dependency of non-key attributes on non-key attributes.
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