| Selection σc1 Λ c2 Λ ... cn(R ) ≡ σc1(σc2(...(σcn (R ))...) All nonspatial relation are moveed to the right σc1(σc2 (R )) ≡ σc2(σc1 (R )) Nonspatial selection is first than spatial selection Projection If ai’ are a set of attributes such that ai c ai+1 for i = 1,...n-1, then πa1 (R ) ≡ πa1(πa2(...(πan (R ))...) Cross Product and Join Conmutativity: R  S ≡ S Associativity R Implication (R Selection, Projection and Join If the selection condition involves attributes used by the projection operator: πa(σc(R )) ≡ σc ( πa (R )) If a condition of selection c involves an attribute that only appears in R and not in S, then: σc(R S ) ≡ σc (R ) Projection can be processed with Join: πa(R where a1 is a subset of a, which appears in R, and a2 is the subset of a that appears in S |
Última modificación: Saturday, 19 de November de 2005, 10:07