TAILIEUCHUNG - Database Systems - Part 10
Tài liệu tham khảo về hệ thống cơ sở dữ liệu giới thiệu về bình thường hóa | COP 4710: Database Systems Spring 2004 Introduction to Normalization BÀI 10, ½ ngày School of Electrical Engineering and Computer Science University of Central Florida Instructor : Mark Llewellyn markl@ CC1 211, 823-2790 Phd, MS, Under If R is a relational schema with attributes A1,A2, ., An and a set of functional dependencies F where X ⊆ {A1,A2,.,An} then X is a key of R if: X → F+, and no proper subset Y ⊆ X gives Y → F+. Basically, this definition means that you must attempt to generate the closure of all possible subsets of the schema of R and determine which sets produce all of the attributes in the schema. Determining the Keys of a Relation Schema Let r = (C, T, H, R, S, G) with F = {C T, HR C, HT R, CS G, HS R} Step 1: Generate (Ai)+ for 1 i n C+ = {CT}, T+ = {T}, H+ = {H} R+ = {R}, S+ = {S}, G+ = {G} no single attribute is a key for R Step 2: Generate (AiAj)+ for 1 i n, 1 j
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