Week7-normalisation.pdf - Normalisation Normalization...

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Normalisation Normalization : branch of relational theory providing design insights. The goals of normalization: be able to characterise the level of redundancy in a relational schema provide mechanisms for transforming schemas to remove redundancy Normalization draws heavily on the theory of functional dependencies 51
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Normal Forms Normalization theory defines six normal forms (NFs). Each normal form: involves a set of dependency properties that a schema must satisfy gives guarantees about presence/absence of update anomalies Higher normal forms have less redundancy less update problems. 52
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Normal Forms Must first decide which normal form rNF is "acceptable". The normalization process: check whether each relation in schema is in rNF if a relation is not in rNF » partition into sub-relations where each is closer to rNF repeat until all relations in schema are in rNF 53
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Normal Forms A brief history of normal forms: First,Second,Third Normal Forms (1NF,2NF,3NF) (Codd 1972) Boyce-Codd Normal Form (BCNF) (1974) Fourth Normal Form (4NF) (Zaniolo 1976, Fagin 1977) Fifth Normal Form (5NF) (Fagin 1979) NF hierarachy: 5NF 4NF BCNF 3NF 2NF 1NF 1NF allows most redundancy; 5NF allows least redundancy. 54
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Normal Forms 55
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Normal Forms In practice, BCNF and 3NF are the most important. (these are generally the "acceptable normal forms" for relational design) Boyce-Codd Normal Form (BCNF): eliminates all redundancy due to functional dependencies but may not preserve original functional dependencies Third Normal Form (3NF): eliminates most (but not all) redundancy due to fd s guaranteed to preserve all functional dependencies 56
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Relational Decomposition The standard transformation technique to remove redundancy: decompose relation R into relations S and T We accomplish decomposition by selecting (overlapping) subsets of attributes forming new relations based on attribute subsets Properties: R = S T, S ∩ T ≠ {} and ideally r(R) = s(S) t(T) We may require several decompositions to achieve acceptable NF. Normalization algorithms tell us how to choose S and T .
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