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Multi-valued Dependencies and Fourth Normal Form

Multi-valued Dependencies and Fourth Normal Form. COSC 6340. Topics Covered. Definition of Multivalued Dependencies Reasoning about MVDs Fourth Normal Form. Motivation. There are schemas that are in BCNF that do not seem to be sufficiently normalized. Stars. name. street. city. title.

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Multi-valued Dependencies and Fourth Normal Form

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  1. Multi-valued Dependencies and Fourth Normal Form COSC 6340

  2. Topics Covered • Definition of Multivalued Dependencies • Reasoning about MVDs • Fourth Normal Form Ch. Eick: 4thNF and MVD's

  3. Motivation • There are schemas that are in BCNF that do not seem to be sufficiently normalized Stars name street city title year C. Fisher 123 Maple Str. Hollywood Star Wars 1977 C. Fisher 5 Locust Ln. Malibu Star Wars 1977 C. Fisher 123 Maple Str. Hollywood Empire Strikes Back 1980 C. Fisher 5 Locust Ln. Malibu Empire Strikes Back 1980 C. Fisher 123 Maple Str. Hollywood Return of the Jedi 1983 C. Fisher 5 Locust Ln. Malibu Return of the Jedi 1983 Ch. Eick: 4thNF and MVD's

  4. Attribute Independence • No reason to associate address with one movie and not another • When we repeat address and movie facts in all combinations, there is obvious redundancy • However, NO BCNF violation in Stars relation • There are no non-trivial FD’s at all, all five attributes form the only superkey • Why? Ch. Eick: 4thNF and MVD's

  5. Multi-valued Dependency Definition:Multivalued dependency (MVD): A1A2…An  B1B2…Bm holds for relation R if: For all tuples t, u in R If t[A1A2...An] = u[A1A2...An], then there exists a v in R such that: (1) v[A1A2...An] = t[A1A2...An] = u[A1A2...An] (2) v[B1B2…Bm] = t[B1B2…Bm] (3) v[C1C2…Ck] = u[C1C2…Ck], where C1C2…Ck is all attributes in R except (A1A2...An B1B2…Bm) Ch. Eick: 4thNF and MVD's

  6. Pictorially Speaking... A’s B’s Others • An MVD guarantees v exists • The existence of a fourth tuple w is implied by interchanging t and u t v u w Ch. Eick: 4thNF and MVD's

  7. C. Fisher 5 Locust Ln. Malibu Star Wars 1977 C. Fisher 123 Maple Str. Hollywood Return of the Jedi 1983 C. Fisher 5 Locust Ln. Malibu Return of the Jedi 1983 Example: name  street city Stars name street city title year C. Fisher 123 Maple Str. Hollywood Star Wars 1977 t v C. Fisher 123 Maple Str. Hollywood Empire Strikes Back 1980 u C. Fisher 5 Locust Ln. Malibu Empire Strikes Back 1980 Ch. Eick: 4thNF and MVD's

  8. C. Fisher 123 Maple Str. Hollywood Return of the Jedi 1983 C. Fisher 5 Locust Ln. Malibu Return of the Jedi 1983 Example: name  street city Stars name street city title year C. Fisher 123 Maple Str. Hollywood Star Wars 1977 u C. Fisher 5 Locust Ln. Malibu Star Wars 1977 w v C. Fisher 123 Maple Str. Hollywood Empire Strikes Back 1980 t C. Fisher 5 Locust Ln. Malibu Empire Strikes Back 1980 Ch. Eick: 4thNF and MVD's

  9. More on MVDs • Intuitively, A1A2…An  B1B2…Bm says that the relationship between A1A2…An and B1B2…Bm is independent of the relationship between A1A2…An and R -{B1B2…Bm} • MVD's uncover situations where independent facts related to a certain object are being squished together in one relation • Functional dependenciesrule out certain tuples from being in a relation • How? • Multivalued dependenciesrequire that other tuples of a certain form be present in the relation • a.k.a. tuple-generating dependencies Ch. Eick: 4thNF and MVD's

  10. Let’s Illustrate • In Stars, we must repeat the movie (title, year) once for each address (street, city) a movie star has • Alternatively, we must repeat the address for each movie a star has made • Example: Stars with namestreet city name street city title year C. Fisher 123 Maple Str. Hollywood Star Wars 1977 C. Fisher 5 Locust Ln. Malibu Empire Strikes Back 1980 C. Fisher 123 Maple Str. Hollywood Return of the Jedi 1983 • Is an incomplete extent of Stars • Infer the existence of a fourth tuple under the given MVD Ch. Eick: 4thNF and MVD's

  11. Trivial MVDs • Trivial MVD A1A2…An  B1B2…Bm where B1B2…Bm is a subset of A1A2…An or (A1A2…An B1B2…Bm ) contains all attributes of R Ch. Eick: 4thNF and MVD's

  12. Reasoning About MVDs • FD-IS-AN-MVD Rule (Replication) If A1A2…An  B1B2…Bm then A1A2…An  B1B2…Bm holds Ch. Eick: 4thNF and MVD's

  13. Reasoning About MVDs • COMPLEMENTATION Rule If A1A2…An  B1B2…Bm then A1A2…An C1C2…Ck where C1C2…Ck is all attributes in R except (A1A2…An B1B2…Bm ) • AUGMENTATION Rule If XY and WZ then WX YZ • TRANSITIVITY Rule If XY and YZ then X  (Z-Y) Ch. Eick: 4thNF and MVD's

  14. Coalescence Rule for MVD X  Y Then:X  Z If:   W:W  Z Remark: Y and W have to be disjoint and Z has to be a subset of or equal to Y Ch. Eick: 4thNF and MVD's

  15. Definition 4NF • Given: relation R and set of MVD's for R • Definition: R is in 4NF with respect to its MVD's if for every non-trivial MVD A1A2…AnB1B2…Bm , A1A2…An is a superkey • Note: Since every FD is also an MVD, 4NF implies BCNF • Example: Stars is not in 4NF Ch. Eick: 4thNF and MVD's

  16. Decomposition Algorithm (1) apply closure to the user-specified FD's and MVD's**: (2) repeat until no more 4NF violations: if R with AA ->> BB violates 4NF then: (2a) decompose R into R1(AA,BB) and R2(AA,CC), where CC is all attributes in R except (AA  BB) (2b) assign FD's and MVD's to the new relations** ** MVD's: hard problem! • No simple test analogous to computing the attribute closure for FD’s exists for MVD’s. You are stuck to have to use the 5 inference rules for MVD’s when computing the closure! Ch. Eick: 4thNF and MVD's

  17. Exercise • Decompose Stars into a set of relations that are in 4NF. • namestreet city is a 4NF violation • Apply decomposition: R(name, street, city) S(name, title, year) • What about namestreet city in R and nametitle year in S? Ch. Eick: 4thNF and MVD's

  18. Exercise For the relation R(A,B,C,D) with only MVD’s AB and AC find all 4NF violations and decompose R into a collection of relation schemas in 4NF. Ch. Eick: 4thNF and MVD's

  19. Solution • Since there are no functional dependencies, the only key is all four attributes, ABCD. • Thus, each of the nontrivial multivalued dependencies A->->B and A->->C violate 4NF. • Separate out the attributes of these dependencies, first decomposing into AB and ACD • Then decompose the latter into AC and AD because A->->C is still a 4NF violation for ACD. • The final set of relations are AB, AC, and AD. Ch. Eick: 4thNF and MVD's

  20. Exercise Suppose we have relation R(A,B,C) with MVD AB. If we know that the tuples (a,b1,c1), (a,b2,c2), and (a,b3,c3) are in the current instance of R, what other tuples do we know must also be in R? Ch. Eick: 4thNF and MVD's

  21. Solution • Since A->->B, and all the tuples have the same value for attribute A, we can pair the B-value from any tuple with the value of the remaining attribute C from any other tuple. • Thus, we know that R must have at least the nine tuples of the form (a,b,c), where b is any of b1, b2, or b3, and c is any of c1, c2, or c3. That is, we can derive, using the definition of a multivalued dependency, that each of the tuples (a,b1,c2), (a,b1,c3), (a,b2,c1), (a,b2,c3), (a,b3,c1), and (a,b3,c2) are also in R. Ch. Eick: 4thNF and MVD's

  22. Another View of 4NF True MVD XY:= non-trivial & XY does not hold ‘s True MVD’s MVD’s that are also FD’s Remark: If XY is a true MVD then X cannot be a superkey (because XY does not hold); Therefore, true MVD’s always violate 4NF (“true MVD’s are always bad) Trivial MVD’s 4NF:= Relation is in BCNF and there are no true MVD’s (yellow part is empty) XY and XY XY and not XY XY Ch. Eick: 4thNF and MVD's

  23. H1-2005-Problem8 8) Normalization [6] graded R(A,B,C,D,E,F) is given with: (1) ABCD (2)CDAB (3)ABF (4) FE • What are the candidate keys of relation R? [1] • b) Transform R into a relational schema that is in BCNF and does not have any lost functional dependencies! [5] Correct Solution: • Candidate keys: AB and CD • Decompose R into R1(A,B,C,D,F) with local FD’s (1), (2), (3) and R2(E,F) with local FD’s (4) Due to the fact that all four dependencies are still present no functional dependency has been lost. Moreover, all functional depencies are good A non-optimal (“too many relations”) solution I also saw was: Decompose R into R1(A,B,C,D) with local functional dependencies ABCD and CDAB, R2(A,B,F) with local functional dependencies ABF and R3(F,E) with local functional dependencies FA.. Ch. Eick: 4thNF and MVD's

  24. Problem 1; H1-2004 • Candidate keys are: {a,b}, {a,d}, {a,e} • 14 superkeys total • All but the first functional dependency are bad Ch. Eick: 4thNF and MVD's

  25. Problem 2; H2-04 • No; EBC is a “true” multi-valued dependency and E is not a candidate key (as a matter of fact {E}+={A,D,E,F}see below) • No (but just mentioning neither E ABC nor E CF holds is not sufficient (e.g. if EABC holds then the decomposition is lossless!) ) --- a counter examples should be given to show that the statement is false! • Yes C is candidate key; therefore CBDEF; therefore CBDEF • Yes E BC and BC BCD implies ED due to MVD-transitivity (CCDBCBCD BCBCD) • Yes EBC; therefore EADF; moreover, CADF and using the Coalescence Rule we obtain EADF; therefore, EA holds • No R is not in BCNF because EADF holds and E is not a candidate key. Ch. Eick: 4thNF and MVD's

  26. Problem 3a-2004 From AB and AC we can infer: ABC?? (1)AC AAC (2) AB  ACABC (3) ACABC  ACDE (4) AAC, ACDE ADE (5) ADE ABC Using: 1. Augmentation, 2.Augmentation, 3.Complementation, 4.Transitivity, 5. Complementation Wrong!! Remark: This problem will be revised in Homework3-2005; it is too complicated to worry about it for the midterm exam! Ch. Eick: 4thNF and MVD's

  27. MDV’s and FD’s --- Ungraded Homework Assume we have a relation R(A,B,C,D,E) with the following dependencies: (1)AB CDE (2)CD  ABE (3)E DB Answer the following questions giving reasons for your answers: a)Is R in BCNF? ????? (answer after Spring break)Warning: The presence of the MVD might imply other functional dependencies (see textbook page 637) b)Does ABE  D hold for R? yes c)Does CD  B hold for R? yes d)Does E D always hold for R (either show that this dependency can be inferred from the given 3 dependencies, or give a counter example of a relation that satisfies (1), (2), (3) but violates ED)? No Ch. Eick: 4thNF and MVD's

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