Multivalued Dependencies ( K- k8 c; ]( S7 W4 U$ B; R7 V" yFourth Normal Form * `) ]1 d9 \: i: A( H2 [Reasoning About FD’s + MVD’s ?, M2 W! ]% t5 e（欢迎访问avcar：https://avcar.vip）
1 6 g; p3 a# f# z; {% q0 ]Definition of MVD; l# ~$ |* X6 s（欢迎访问avcar：https://avcar.vip）
A multivalued dependency (MVD) on 2 b3 ]# j! y( Q" t0 j/ ^1 q（欢迎访问avcar：https://avcar.vip）
R, X ->->Y , says that if two tuples of R & _4 ?2 ] u" bagree on all the attributes of X, then + @* W# T7 z% W6 Ctheir components in Y may be ) A' ^$ S/ t& |. M* cswapped, and the result will be two 9 c$ x i: f, Atuples that are also in the relation.8 n/ ?% E. n8 W（欢迎访问avcar：https://avcar.vip）
i.e., for each value of X, the values of Y8 P4 _* t! j! C（欢迎访问avcar：https://avcar.vip）
are independent of the values of R-X-Y. 2 # [4 x+ i/ ? z O; O8 p3 - C; P/ L& q7 |: qExample: MVD % O& }% [3 h* V6 YDrinkers(name, addr, phones, beersLiked) + }/ {- ]7 Z; ^3 ?/ P% yA drinker’s phones are independent of K/ _0 Q u$ _2 _5 g! z- Q（欢迎访问avcar：https://avcar.vip）
the beers they like." S8 P- |, B, |8 S x0 n$ G' e（欢迎访问avcar：https://avcar.vip）
name->->phones and name ->- 6 o. t+ W+ o1 Z: W6 V>beersLiked." r* D' q8 [7 [ R! T（欢迎访问avcar：https://avcar.vip）
Thus, each of a drinker’s phones appears 6 P& z$ U5 p* M! t) gwith each of the beers they like in all * X% W" Y* r! e5 D- gcombinations.( v# x8 Y& P0 F* j8 V4 j2 B（欢迎访问avcar：https://avcar.vip）
This repetition is unlike FD redundancy. 5 x& Y9 p) H: k1 Nname->addr is the only FD. 9 I5 f2 q5 e5 d* D4$ @+ M. f3 w* ~5 B% z+ p, C（欢迎访问avcar：https://avcar.vip）
Tuples Implied by name->->phones $ Q5 z) \8 ^8 N, EIf we have tuples: ! ~( W1 `/ o; d3 E# r8 E; _name addr phones beersLiked 1 m& b$ [3 h; Y/ K, xsue a p1 b1 R5 A* w) y5 y1 u' b: c7 A0 Nsue a p2 b2 ' v6 D- T, R3 Ysue a p2 b1: i2 k1 E7 F: ?! x4 z（欢迎访问avcar：https://avcar.vip）
sue a p1 b2 1 P' s8 _4 S8 X4 |Then these tuples must also be in the relation.+ l) _9 ] T$ Y/ m（欢迎访问avcar：https://avcar.vip）
5 6 I, h, |* h' M8 `% |! o9 oPicture of MVD X ->->Y X Y others ' g' B: N0 g: k* Lequal7 x- p) q; ]7 { R: z（欢迎访问avcar：https://avcar.vip）
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发表于 2020-4-9 10:49:29