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Closure of an attribute x is the set of all attributes that are functional dependencies on X with respect to F. It is denoted by X^{+} which means what X can determine.

Let’s see the algorithm to compute X^{+}

- Step 1 − X
^{+}=X - Step 2 − repeat until X
^{+}does not change- For each FD Y->Z in F
- If Y ⊆ X
^{+}then X^{+}= X^{+}U Z

- If Y ⊆ X

- For each FD Y->Z in F

Consider a relation R(A,B,C,D,E,F)

F: E->A, E->D, A->C, A->D, AE->F, AG->K.

Find the closure of E or E^{+}

The closure of E or E+ is as follows −

E^{+}= E =EA {for E->A add A} =EAD {for E->D add D} =EADC {for A->C add C} =EADC {for A->D D already added} =EADCF {for AE->F add F} =EADCF {for AG->K don’t add k AG ⊄ D^{+})

Let the relation R(A,B,C,D,E,F)

F: B->C, BC->AD, D->E, CF->B. Find the closure of B.

The closure for B is as follows −

B^{+} = {B,C,A,D,E}

Closure is used to find the candidate keys of R and compute F^{+}

Candidate key of R: X is a candidate keys of R if X->{R}

For example,

R(A,B,C,D,E,F) WHERE F:A->BC, B->D, C->DE, BC->F. Then, find the candidate keys of R.

A^{+}= {A,B,C,D,E,F}={R}=>A is a candidate key

B^{+}= {B,D} => B is not a candidate key

C^{+}= {C,D,E} => C is not a candidate key

BC^{+}= {B,C,D,E,F} => BC is not a candidate key

Closure of F (F+): F+ is the set of all FDs that can be inferred/ derived from F. Using Armstrong Axioms repeatedly on F, we can compute all the FDs.

R(A,B,C,D,E) AND F: A->B,B->C, C->D, A->E. Find the closure of F

A^{+}= {A,B,C,D,E}

B^{+}= {B,C,D}

C^{+}= {C,D}

F^{+}= {A->A, A->B, A->C, A->D, A->E, B->B, B->C, B->D, C->C, C->D}

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