Chapter14-FunctionalDependencies.pptx

Fds are derived from the real world constraints on

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FDs are derived from the real-world constraints on the attributes The arrow means “functionally determines;” the tuple on the right is functionally dependent Functional Dependencies and Normalization 18
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Examples of FD Constraints Social security number determines employee name SSN ENAME Project number determines project name and location PNUMBER {PNAME, PLOCATION} Employee ssn and project number determines the hours per week that the employee works on the project {SSN, PNUMBER} HOURS Functional Dependencies and Normalization 19
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Examples of FD Constraints An FD is a property of the attributes in the schema R The constraint must hold on every relation instance r(R) If K is a key of R, then K functionally determines all attributes in R (since we never have two distinct tuples with t1[K]=t2[K]) Functional Dependencies and Normalization 20
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Ruling Out FDs Note that given the state of the TEACH relation, we can say that the FD: Text → Course may exist. However, the FDs Teacher → Course, Teacher → Text and Couse → Text are ruled out. Functional Dependencies and Normalization 21
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What FDs May Exist? A relation R (A, B, C, D) with its extension. Which FDs may exist in this relation? Functional Dependencies and Normalization 22
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Normal Forms Based on Primary Keys Normalization of Relations Practical Use of Normal Forms Definitions of Keys and Attributes Participating in Keys First Normal Form Second Normal Form Third Normal Form Functional Dependencies and Normalization 23
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Normalization of Relations Normalization: The process of decomposing unsatisfactory "bad" relations by breaking up their attributes into smaller relations Normal form: Condition using keys and FDs of a relation to certify whether a relation schema is in a particular normal form Functional Dependencies and Normalization 24
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Normalization of Relations 2NF, 3NF, BCNF based on keys and FDs of a relation schema 4NF based on keys, multi-valued dependencies : MVDs; 5NF based on keys, join dependencies : JDs Additional properties may be needed to ensure a good relational design (lossless join, dependency preservation; see Chapter 15) Functional Dependencies and Normalization 25
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Practical Use of Normal Forms Normalization is carried out in practice so that the resulting designs are of high quality and meet the desirable properties The practical utility of these normal forms becomes questionable when the constraints on which they are based are hard to understand or to detect The database designers need not normalize to the highest possible normal form (usually up to 3NF and BCNF. 4NF rarely used in practice.) Denormalization : The process of storing the join of higher normal form relations as a base relation—which is in a lower normal form Functional Dependencies and Normalization 26
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Definitions of Keys and Attributes Participating in Keys A superkey of a relation schema R = {A1, A2, ....
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  • Fall '09
  • SUNANHAN
  • Database normalization, BCNF

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