# F_D1 - Functional Dependencies Overview Basic Definitions...

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Functional Dependencies

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Overview b Basic Definitions b Trivial and nontrivial dependencies b Closure of a set of dependencies b Closure of a set of attributes b Irreducible sets of dependencies
Functional Dependencies b A functional dependency is basically a many-to-one relationship from one set of attributes to another within a given relvar. b Eg: s {Supplier_Number,Part_Number} B {Quantity}

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Basic definitions b Two cases: b Case a: s the value of a given relvar at a given point in time. b Case b: s the set of all possible values that the given relvar might assume at different time SCP
Basic definitions b Functional dependence, Case a: b Let r be a relation, and let X and Y be arbitrary subsets of the set of attributes of r . b Then we say that Y is functionally dependent on X – in symbols, s if and only if each X value in r has associated with it precisely one Y value in r . s Whenever two tuples of r agree on their X value,they also agree on Y values. s Ex: { SUPPLIER_NUMBER } -> { CITY }

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b Also satisfies several FDs: { SUPPLIER_NUMBER, PART_NUMBER } -> { CITY } { SUPPLIER_NUMBER, PART_NUMBER }->{CITY,QUANTITY} { SUPPLIER_NUMBER } -> { QUANTITY } { QUANTITY } -> { SUPPLIER_NUMBER } { SUPPLIER_NUMBER, PART_NUMBER } -> {SUPPLIER_NUMBER} b Left and right sides of an FD are called the determinant and the dependent respectively.
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F_D1 - Functional Dependencies Overview Basic Definitions...

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