Hendrolim's Weblog

December 23, 2007

Tips2 Functional Dependency

Filed under: Database — hendrolim @ 5:25 pm
tips2 dlm FD
1. Semua F+ dapat dicari dengan Armstrong’s Axioms:
– if b C= a, then a -> b (reflexivity)
– if a -> b, then xa -> xb (augmentation)
– if a -> b, and b -> c, then a -> c (transitivity)

2. rules tambahan
– If a -> b and a -> c, then a -> bc (union)
– If a -> bc, then a -> b and a -> c (decomposition)
– If a -> b and xb -> d, then ax -> d (pseudotransitivity)

3. Testing :
-. a adl super key, jika a+ mengandung semua atribut R.
-. a adl candidate key, jika a super key dan a minimal
-. a -> b adl FD, jika b C= a+.

4. Canonical cover dari F adl himpunan FD yg minimal dan tidak memiliki redundansi FD atau atribut extra.

5. Fc adl canonical cover dari F jika
-. F dapat menurunkan Fc.
-. Fc dapat menurunkan F.
-. Tidak ada atribut extra pada Fc.
-. Setiap ruas kiri di Fc unik.

6. Testing untuk atribut extra.
Misalkan a -> b pada F
-. atribut x E a adl extra jika (a-x)+ pada F mengandung x
-. atribut x E b adl extra jika a+ pada F’ mengandung x dimana F’=(F-(a -> b))U(a -> (b-x))

7. Algoritma mencari canonical cover dari F :
Use the union rule to replace any dependencies in F
a -> b and a -> c with a -> bc
Find a functional dependency a -> b with an
extraneous attribute either in a or in b
If an extraneous attribute is found, delete it from a -> b
until F does not change


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