ics321-20091117-fdnf2

ics321-20091117-fdnf2 - ICS 321 Fall 2009 Schema Refinement...

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ICS 321 Fall 2009 Schema Refinement Asst. Prof. Lipyeow Lim University of Hawaii at Manoa 11/17/2009 1 Lipyeow Lim -- University of Hawaii at Manoa

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Two More Rules Union If X Y and X Z , then X YZ Eg. FLD A and FLD T, then FLD AT Decomposition If X YZ , then X Y and X Z Eg. FLD AT , then FLD A and FLD T Trivial FDs Right side is a subset of Left side Eg. F F, FLD FD 11/17/2009 Lipyeow Lim -- University of Hawaii at Manoa 2 Firstname Lastname DOB Address Telephone John Smith Sep 9 1979 Honolulu,HI 808-343-0809
Implication : An FD f is implied by a set of FDs F if f holds whenever all FDs in F hold. f=A C is implied by F={ A B, B C} (using Armstrong’s transitivity) Closure F + : the set of all FDs implied by F Algorithm : start with F + =F keep adding new implied FDs to F + by applying the 5 rules ( Armstrong’s Axioms + union + decomposition) Stop when F + does not change anymore. 11/17/2009

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ics321-20091117-fdnf2 - ICS 321 Fall 2009 Schema Refinement...

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