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12.1 Introduction (1/3)

12.1 Introduction (1/3). Redundancy (See Fig. 12.1) A good design principle is “one fact in one place” The principles of further normalization allow us to recognize the redundancy and to replace the relvars in question by ones that are more desirable in some way.

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12.1 Introduction (1/3)

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  1. 12.1 Introduction (1/3) • Redundancy (See Fig. 12.1) • A good design principle is “one fact in one place” • The principles of further normalization allow us to recognize the redundancy and to replace the relvars in question by ones that are more desirable in some way. e.g., {S#,City,P#,Qty} {S#,City}, {S#,P#,Qty} Advanced Database System

  2. 12.1 Introduction (2/3) • Normal forms A relvar is said to be in a particular normal form if it satisfies a certain prescribed set of conditions. • The normalization procedure is reversible; it means the normalization process is nonloss or information-preserving. • Codd: 1NF, 2NF, 3NF, BCNF • Fagin: 4NF, PJ/NF (5NF) (See Fig. 12.2) Advanced Database System

  3. 12.1 Introduction (3/3) • Two remarks: 1. Normalization is a useful aid in the process, but it is not a panacea. 2. The ideas of normalization can then be used to verify that the resulting design does not unintentionally violate any of the normalization principles. Advanced Database System

  4. 12.2 Nonloss Decomposition and Functional Dependencies • Whether a given decomposition is nonloss is intimately bound up with the concept of functional dependence. • Decomposition is actually a process of projection. • Example (See Fig. 12.3) • The reversibility means that the original relvar is equal to the join of its projections. Advanced Database System

  5. 12.2 Nonloss Decomposition and Functional Dependencies (Cont.) • Heath’s theorem: Let R{A,B,C} be a relvar, where A, B, and C are sets of attributes. If R satisfies the FD A →B, then R is equal to the join of its projections on {A,B} and {A,C}. • In case (b), one of the FDs is lost in the decomposition. FD S#→City • More on functional dependencies: • 1. Irreducibility • {S#,P#}→City • 2. FD diagrams • (See Fig. 12.4) 3. FDs are a semantic notion A special kind of integrity constraint Advanced Database System

  6. 12.3 First, Second, and Third Normal Forms (1/6) • We assume for simplicity that each relvar has exactly one candidate key, which we further assume is the primary key. • Third normal form (very informal definition): A relvar is in 3NF if and only if the nonkey attributes are both: (a) Mutually independent (b) Irreducibly dependent on the primary key. • First normal form: A relvar is in 1NF if and only if, in every legal value of that relvar, every tuple contains exactly one value for each attribute. • Example (See Fig. 12.5 & 12.6) The primary key of FIRST is {S#,P#}. Advanced Database System

  7. Fig. 12.5 & 12.6 Advanced Database System

  8. 12.3 First, Second, and Third Normal Forms (2/6) • Relvar FIRST violates both conditions a and b in our preliminary 3NF definition. • Update anomalies corresponding to the FD S#→City • After decomposition SECOND(S#,Status,City), SP(S#,P#,Qty) (See Fig. 12.7 & 12.8) Advanced Database System

  9. Fig. 12.7 & 12.8 Advanced Database System

  10. 12.3 First, Second, and Third Normal Forms (3/6) • The effect of the decomposition of FIRST into SECOND and SP is to eliminate the dependencies that are not irreducible. • Second normal form: A relvar is in 2NF if and only if it is in 1NF and every nonkey attribute is irreducibly dependent on the primary key. • In summary R{A,B,C,D} Primary Key {A,B} A →D R1{A,D} Primary Key {A} R2{A,B,C} Primary Key {A,B} Foreign Key {A} References R1 Advanced Database System

  11. 12.3 First, Second, and Third Normal Forms (4/6) • Transitive: If A→B and B→C, then A→C • Update anomalies corresponding to the FD City →Status • After decomposition SC(S#,City), CS(City, Status) (See Fig. 12.9 & 12.10) Advanced Database System

  12. 12.3 First, Second, and Third Normal Forms (5/6) • The effect of the further decomposition is to eliminate the transitive dependencies of STATUS on S#. • Third normal form: A relvar is in 3NF if and only if it is in 2NF and every nonkey attribute is nontransitively dependent on the primary key. Advanced Database System

  13. 12.3 First, Second, and Third Normal Forms (6/6) • In summary R{A,B,C} Primary Key {A} B →C R1{B,C} Primary Key {B} R2{A,B} Primary Key {A} Foreign Key {B} References R1 • It is not possible just to look at the value at a given time and to say whether the relvar is in 3NF. • Even knowing the dependencies, it is never possible to prove by examining a given value that the relvar is in 3NF. Advanced Database System

  14. 12.4 Dependency Preservation (1/3) • A given relvar can be nonloss-decomposed in a variety of different ways. • Example (See Fig. 12.11) Decomposition A: SC{S#,City} and CS{City,Status} Decomposition B: SC{S#,City} and SS{S#,Status} Advanced Database System

  15. 12.4 Dependency Preservation (2/3) • Decomposition B is less satisfactory than decomposition A. It is still not possible in B to insert the information that a particular city has a particular status unless some supplier is located in that city. • In decomposition A, the two projections are independent of one another. • In decomposition B, updates to either of the two projections must be monitored to ensure that the FD City →Status is not violated. Advanced Database System

  16. 12.4 Dependency Preservation (3/3) • A guideline for independent projections: Rissanen shows that projections R1 and R2 of a relvar R are independent if and only if both of the following are true: 1. Every FD in R is a logical consequence of those in R1 and R2. 2. The common attributes of R1 and R2 form a candidate key for at least one of the pair. • A relvar that cannot be decomposed into independent projections is called atomic. • The fact that some given relvar is not atomic should not necessarily be taken to mean that it should be decomposed into atomic components. Advanced Database System

  17. 12.5 Boyce/Codd Normal Form (1/7) • Considering the case of a relvar that 1. Had two or more candidate keys, such that 2. The candidate keys were composite, and 3. They overlapped (i.e., had at least one attribute in common) • Boyce/Codd normal form: A relvar is in BCNF if and only if every nontrivial, left-irreducible FD has a candidate key as its determinant. • In other words, the only arrows in the FD diagram are arrows out of candidate keys. e.g., FIRST is not in BCNF determinants S#, City, {S#,P#} candidate keys {S#,P#} SECOND is not in BCNF determinants S#, City candidate keys S# SP, SC, and CS are each in BCNF Advanced Database System

  18. 12.5 Boyce/Codd Normal Form (2/7) • Example: Two disjoint candidate keys S{S#,Sname,Status,City} BCNF (See Fig. 12.12) Advanced Database System

  19. 12.5 Boyce/Codd Normal Form (3/7) • Example: Overlapping candidate keys SSP{S#,Sname,P#,Qty} determinants S#, Sname, {S#,P#}, {Sname,P#} candidate keys {S#,P#}, {Sname,P#} It is not in BCNF, but is in 3NF. (See Fig. 12.13) Two projections: 1. SS{S#,Sname} and SP{S#,P#,Qty} 2. SS{S#,Sname} and SP{Sname,P#,Qty} Advanced Database System

  20. 12.5 Boyce/Codd Normal Form (4/7) • Example: Overlapping candidate keys SJT{S,J,T} The constraints: 1. For each subject, each student of that subject is taught by only one teacher. 2. Each teacher teaches only one subject (but each subject is taught by several teachers). (See Fig. 12.14 & 12.15) Advanced Database System

  21. 12.5 Boyce/Codd Normal Form (5/7) T is a determinant but not a candidate key. not in BCNF Two projections: ST{S,T} and TJ{T,J} But they are not independent. The FD {S,J}→T cannot be deduced from the FD T →J. Inserting a tuple for Smith and Prof. Brown into relvar ST reject Advanced Database System

  22. 12.5 Boyce/Codd Normal Form (6/7) • The twin objectives of (a) decomposing a relvar into BCNF components, and (b) decomposing it into independent components can occasionally be in conflict. • Relation SJT is atomic in Rissanen’s sense, even though it is not in BCNF. • The fact that an atomic relvar cannot be decomposed into independent components does not mean that it cannot be decomposed at all. Advanced Database System

  23. 12.5 Boyce/Codd Normal Form (7/7) • Example: Overlapping candidate keys EXAM{S,J,P} The constraint: There are no ties; that is, no two students obtained the same position in the same subject. (See Fig. 12.16) Two candidate keys: {S,J}, {J,P} BCNF Advanced Database System

  24. 12.6 A Note on Relation-Valued Attributes(1/3) • Asymmetric Advanced Database System

  25. 12.6 A Note on Relation-Valued Attributes(2/3) • Example: 1. Get S# for suppliers who supply part P1 2. Get P# for parts supplied by supplier S1 (SPQ Where Tuple {P# P#(‘P1’)}  PQ {P#}) {S#} ((SPQ Where S# = S# (‘S1’)) Ungroup PQ) {P#} • Example: 1. Create a new shipment for supplier S6, part P5, quantity 500 2. Create a new shipment for supplier S2, part P5, quantity 500 Insert SPQ Relation { Tuple { S# S# (‘S6’), PQ Relation { Tuple { P# P#(‘P5’), Qty Qty (500) } } } }; Update SPQ Where S# = S# (‘S2’) { Insert PQ Relation { Tuple { P# P#(‘P5’), Qty Qty (500) } } }; Advanced Database System

  26. 12.6 A Note on Relation-Valued Attributes(3/3) • However this position should be seen as a guideline only, not as an inviolable law. Var RVK Base Relation { Rvname Name, Ck Relation { Attrname Name } } Key { Rvname, Ck }; (See Fig. 12.18) Advanced Database System

  27. Fig. 12.18 Advanced Database System

  28. The End. Advanced Database System

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