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What is Functional Dependency in Normalization?
Functional dependencies (FDs) are used to specify formal measures of the "goodness" of relational designs and used to define normal forms for relations. FDs are constraints that are derived from the meaning and interrelationships of the data attributes. FDs are derived from the real-world constraints on the attributes
For example: A set of attributes X functionally determines a set of attributes Y if the value of X determines a unique value for Y 
X-->Y holds if whenever two tuples have the same value for X, they must have the same value for Y If t1[X] =t2[X], then t1[Y] =t2[Y] in any relation instance r(R)
X-->Y in R specifies a constraint on all relation instances r(R)