Multivalued Dependencies 2 _. v+ x k( n6 cFourth Normal Form 5 s/ O0 A0 C8 o; u' k% p8 wReasoning About FD’s + MVD’s: C% a7 u& }: N# K, d$ V/ m（欢迎访问avcar：https://avcar.vip）
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Definition of MVD- N3 u3 v* J) Q. P) T0 |% Z0 ^3 P（欢迎访问avcar：https://avcar.vip）
A multivalued dependency (MVD) on 8 h& m/ H0 h2 u1 W（欢迎访问avcar：https://avcar.vip）
R, X ->->Y , says that if two tuples of R # e. q& x1 J$ A3 o9 y: cagree on all the attributes of X, then * l; m' d2 I0 G( {6 V7 Ntheir components in Y may be % Z2 ^, ^) r- P. d, G' \swapped, and the result will be two , R1 {4 r! ]* U& g' @+ R- [1 Ptuples that are also in the relation. " G8 \, H9 R2 Y9 }i.e., for each value of X, the values of Y7 L( Q& R' m3 H（欢迎访问avcar：https://avcar.vip）
are independent of the values of R-X-Y. 2 5 h7 D3 a& ~% R6 S3 ) p+ t8 L2 d" r6 o+ bExample: MVD * }( a% H: r" `# V, ], x3 ODrinkers(name, addr, phones, beersLiked): }7 J* r" ]3 @- w3 [（欢迎访问avcar：https://avcar.vip）
A drinker’s phones are independent of ; O' ?! ^. f! e（欢迎访问avcar：https://avcar.vip）
the beers they like.% t/ X+ F& }7 E M. L! Y8 r/ i/ M（欢迎访问avcar：https://avcar.vip）
name->->phones and name ->-- ? m3 S2 K$ F+ ^+ i（欢迎访问avcar：https://avcar.vip）
>beersLiked. % g$ a7 D( m7 r% G( _# T2 bThus, each of a drinker’s phones appears $ g4 u5 |$ B" E9 w8 H2 o D4 j（欢迎访问avcar：https://avcar.vip）
with each of the beers they like in all + p9 ^' E1 S/ G+ m; d4 U# B$ ucombinations. 1 {# n# o& r1 |& E% k% B9 KThis repetition is unlike FD redundancy.0 o2 s5 ^' L6 \7 m5 K" ~（欢迎访问avcar：https://avcar.vip）
name->addr is the only FD.3 [9 y6 k- l0 V（欢迎访问avcar：https://avcar.vip）
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Tuples Implied by name->->phones $ t; A0 F5 |- j; M( d$ U% o# x8 UIf we have tuples: 4 B* ~6 }+ }" A6 Kname addr phones beersLiked - g2 V+ w* P0 C4 E( rsue a p1 b1 " k) C0 i8 K% U" Csue a p2 b2( X8 J- O% T( i% N Q: [（欢迎访问avcar：https://avcar.vip）
sue a p2 b1 # Z5 p2 |4 t# e: i: b9 msue a p1 b2 # \! d; I; S1 r+ ~' {Then these tuples must also be in the relation.& v( |5 p- h$ S（欢迎访问avcar：https://avcar.vip）
5 0 [9 y/ _0 h' @0 oPicture of MVD X ->->Y X Y others 8 u8 u8 Q/ P+ q+ K' r! M& _equal p) R9 e! @2 M7 i4 d（欢迎访问avcar：https://avcar.vip）
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发表于 2020-3-26 13:46:25
