1 / 8

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.

mio
Télécharger la présentation

Relational Design Theory

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related