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Relational Design Theory

Relational Design Theory. Shortcomings of BCNF/4NF. Shortcomings of BCNF/4NF. Boyce- Codd Normal Form Relation R with FDs is in BCNF if: For each A  B , A is a key Fourth Normal Form Relation R with MVDs is in 4NF if: For each nontrivial A B , A is a key.

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Relational Design Theory

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  1. Relational Design Theory Shortcomings of BCNF/4NF

  2. Shortcomings of BCNF/4NF Boyce-Codd Normal Form Relation R with FDs is in BCNF if: For eachA  B,Ais a key Fourth Normal Form Relation R with MVDs is in 4NF if: For each nontrivialA B,Ais a key

  3. Shortcomings of BCNF/4NF Example: College application info. Apply(SSN,cName,date,major) Can apply to each college once for one major Colleges have non-overlapping application dates FDs: Keys: BCNF: Good design?

  4. Shortcomings of BCNF/4NF Example #2 Student(SSN,HSname,GPA,priority) Multiple HS okay, priority determined from GPA FDs: Keys: BCNF: Good design?

  5. Shortcomings of BCNF/4NF Boyce-Codd Normal Form Relation R with FDs is in BCNF if: For eachA  B,Ais a key Fourth Normal Form Relation R with MVDs is in 4NF if: For each nontrivialA B,Ais a key After decomposition, no guarantee dependencies can be checked on decomposed relations

  6. Shortcomings of BCNF/4NF Example #3 Scores(SSN,sName,SAT,ACT) Multiple SATs and ACTs allowed All queries return name + composite score for SSN FDs + keys: MVDs: 4NF: “Denormalized” relation

  7. Shortcomings of BCNF/4NF Example #4 College(cName,state) CollegeSize(cName,enrollment) CollegeScores(cName,avgSAT) CollegeGrades(cName,avgGPA)    BCNF/4NF? Good Design? “Too decomposed”

  8. Shortcomings of BCNF/4NF Designing a database schema • Usually many designs possible • Some are (much) better than others! • How do we choose? • Very nice theory for relational database design • Normal forms  “good” relations • Design by decomposition • Usually intuitive and works well • Some shortcomings • Dependency enforcement • Query workload • Over-decomposition

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