无法显示该图片。 Chapter 7: Relational Database Design
Chapter 7: Relational Database Design
Chapter 7: Relational Database Design First normal form Pitfalls in Relational Database Design Functional Dependencies Decomposition Boyce-Codd Normal Form Third normal form Multivalued Dependencies and Fourth Normal Form Overall Database Design Process 标 Database System Concepts 7.2 OSilberschatz. Korth and Sudarshan
Database System Concepts 7.2 ©Silberschatz, Korth and Sudarshan Chapter 7: Relational Database Design First Normal Form Pitfalls in Relational Database Design Functional Dependencies Decomposition Boyce-Codd Normal Form Third Normal Form Multivalued Dependencies and Fourth Normal Form Overall Database Design Process
First Normal Form Domain is atomic if its elements are considered to be indivisible units Examples of non-atomic domains Set of names, composite attributes Suppose that students are given roll numbers which are strings of the form CS0012 or EE1127 A relational schema is in first normal form(第一范式, 1NF) if the domains of all attributes of r are atomic 标 Database System Concepts 7.3 OSilberschatz. Korth and Sudarshan
Database System Concepts 7.3 ©Silberschatz, Korth and Sudarshan First Normal Form Domain is atomic if its elements are considered to be indivisible units Examples of non-atomic domains: Set of names, composite attributes Suppose that students are given roll numbers which are strings of the form CS0012 or EE1127 A relational schema R is in first normal form（第一范式， 1NF） if the domains of all attributes of R are atomic
Pitfalls in Relational Database Design Relational database design requires that we find a good" collection of relation schemas. a bad design may lead to Repetition of Information Inability to represent certain information Consider the relation schema Lending-schema=(branch-name, branch-city, assets, customer-name, loan-number, amount customer- loan branch-name branch-cit assets name number amount Downtown Brooklyn 9000000 Jones L171000 Redwood Palo Alto 2100000 Smith L-23 2000 Perryridge Horseneck 1700000 Hayes L-15 1500 Downtown Brooklyn 9000000 Jackson L-14 1500 Database System Concepts 7.4 OSilberschatz. Korth and Sudarshan
Database System Concepts 7.4 ©Silberschatz, Korth and Sudarshan Pitfalls in Relational Database Design Relational database design requires that we find a “good” collection of relation schemas. A bad design may lead to Repetition of Information. Inability to represent certain information. Consider the relation schema: Lending-schema = (branch-name, branch-city, assets, customer-name, loan-number, amount)
Example Redundancy(元余): Data for branch-name, branch-city, assets are repeated for each loan that a branch makes Wastes space Complicates updating, introducing possibility of inconsistency of assets value Null values Cannot store information about a branch if no loans exist Can use null values, but they are difficult to handle 标 Database System Concepts 7.5 OSilberschatz. Korth and Sudarshan
Database System Concepts 7.5 ©Silberschatz, Korth and Sudarshan Example Redundancy（冗余）: Data for branch-name, branch-city, assets are repeated for each loan that a branch makes Wastes space Complicates updating, introducing possibility of inconsistency of assets value Null values Cannot store information about a branch if no loans exist Can use null values, but they are difficult to handle
Decomposition Decompose the relation schema Lending schema into Branch-schema=(branch-name, branch-city, assets) Loan-info-schema =(customer-name, loan-number branch-name, amount) All attributes of an original schema(R)must appear in the decomposition(R1, R2) R=R∪R Lossless-join decomposition(无损连接分解) For all possible relations r on schema R r=R1()IR2( 标 Database System Concepts 7.6 @Silberschatz, Korth and Sudarshan
Database System Concepts 7.6 ©Silberschatz, Korth and Sudarshan Decomposition Decompose the relation schema Lending-schema into: Branch-schema = (branch-name, branch-city,assets) Loan-info-schema = (customer-name, loan-number, branch-name, amount) All attributes of an original schema (R) must appear in the decomposition (R1 , R2 ): R = R1 R2 Lossless-join decomposition（无损连接分解）. For all possible relations r on schema R r = R1 (r) R2 (r)
Example of Non Lossless Join Decomposition Decomposition of R=(A, B) R2=(A)R2=(B) A B A B a2 2 B ∏(r) ∏A(r)∏B(r) aaββ 2 2 Database System Concepts 7.7 @Silberschatz, Korth and Sudarshan
Database System Concepts 7.7 ©Silberschatz, Korth and Sudarshan Example of Non Lossless-Join Decomposition Decomposition of R = (A, B) R2 = (A) R2 = (B) A B 1 2 1 A B 1 2 r A(r) B(r) A (r) B (r) A B 1 2 1 2
Goal- Devise a Theory for the Following Decide whether a particular relation R is in good form In the case that a relation R is not in good form, decompose it into a set of relations (R, R2, ...,Rny such that each relation is in good form the decomposition is a lossless-join decomposition Our theory is based on functional dependencies multivalued dependencies Database System Concepts 7.8 @Silberschatz, Korth and Sudarshan
Database System Concepts 7.8 ©Silberschatz, Korth and Sudarshan Goal — Devise a Theory for the Following Decide whether a particular relation R is in “good” form. In the case that a relation R is not in “good” form, decompose it into a set of relations {R1 , R2 , ..., Rn } such that each relation is in good form the decomposition is a lossless-join decomposition Our theory is based on: functional dependencies multivalued dependencies
Functional Dependencies 函救依赖 Constraints on the set of legal relations Require that the value for a certain set of attributes determines uniquely the value for another set of attributes A functional dependency is a generalization of the notion of a key. 标 Database System Concepts 7.9 @Silberschatz, Korth and Sudarshan
Database System Concepts 7.9 ©Silberschatz, Korth and Sudarshan Functional Dependencies （函数依赖） Constraints on the set of legal relations. Require that the value for a certain set of attributes determines uniquely the value for another set of attributes. A functional dependency is a generalization of the notion of a key
Functional Dependencies(Cont.) Let r be a relation schema α CR and Bc r The functional dependency 0→>B holds on R if and only if for any legal relations r(R) whenever any two tuples t, and t2 of r agree on the attributes a, they also agree on the attributes B. That is t1[o]=t2[]→t1[/]=t2[6] K is a superkey for relation schema R if and only if K>R K is a candidate key for R if and only if K→)R.and for no ac K.a→R Database System Concepts 7.10 OSilberschatz. Korth and Sudarshan
Database System Concepts 7.10 ©Silberschatz, Korth and Sudarshan Functional Dependencies (Cont.) Let R be a relation schema R and R The functional dependency → holds on R if and only if for any legal relations r(R), whenever any two tuples t1 and t2 of r agree on the attributes , they also agree on the attributes . That is, t1 [] = t2 [] t1 [ ] = t2 [ ] K is a superkey for relation schema R if and only if K → R K is a candidate key for R if and only if K → R, and for no K, → R