Database System Concepts
Database System Concepts
7th Edition
ISBN: 9789332901384
Author: Henry F. Korth & S. Sudarshan Abraham Silberschatz
Publisher: MCGRAW-HILL HIGHER EDUCATION
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Chapter 7, Problem 16PE

Explanation of Solution

Ensuring a dependency-preserving decomposition into 3NF is a lossless decomposition:

  • Let F be a set of functional dependencies which hold on a schema R.
  • Let sigma={R1,R2,...,Rn} be a dependency-preserving 3NF decomposition of R.
  • Let X be a candidate key for R.
  • Consider a legal instance r of R. Let j=ProjectX(r) join ProjectR1(r) join ProjectR2(r) ... Join ProjectRn(r).
    1. ○ It is required to prove that r=j.
    2. ○ It is claimed that, t1 and t2 are two tuples, where t1=t2 such that t1[X]=t2[X].
    3. ○ The inductive argument is used for proving this claim.
    4. ○ Let F'=F1 union F2 union ... union Fn, where each Fi is the restriction of F to the schema Ri in sigma. Closure of X is computed under F'.
    5. ○ Induction is applied the number of times that the for loop in the algorithm executes.

Basis:

In the first step of the algorithm, result is assigned to X, and hence given that t1[X]=t2[X], it is known that t1[result]=t2[result] is true.

Induction Step:

  • Let t1[result]=t2[result] be true at the end of the kth execution of the for loop...

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