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Database Systems: Design, Implementation, and Management Eighth Edition

Database Systems: Design, Implementation, and Management Eighth Edition. Chapter 3 The Relational Database Model. Objectives. That the relational database model offers a logical view of data About the relational model’s basic component: relations

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Database Systems: Design, Implementation, and Management Eighth Edition

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  1. Database Systems: Design, Implementation, and ManagementEighth Edition Chapter 3 The Relational Database Model

  2. Objectives • That the relational database model offers a logical view of data • About the relational model’s basic component: relations • That relations are logical constructs composed of rows (tuples) and columns (attributes) • That relations are implemented as tables in a relational DBMS • About relational database operators, the data dictionary, and the system catalog • How data redundancy is handled in the relational database model • Why indexing is important 2 Database Systems, 8th Edition

  3. 3.1 A Logical View of Data • Relational model • View data logically rather than physically • Table • Structural and data independence • Resembles a file conceptually • Relational database model easier to understand than hierarchical and network models 3 Database Systems, 8th Edition

  4. Tables and Their Characteristics • Logical view of relational database based on relation • Relation thought of as a table • Table: two-dimensional structure composed of rows and columns • Persistent representation of logical relation • Contains group of related entities = an entity set 4 Database Systems, 8th Edition

  5. 5 Database Systems, 8th Edition Primary key

  6. 6 Database Systems, 8th Edition

  7. 3.2 Keys • Each row in a table must be uniquely identifiable • Key is one or more attributes that determine other attributes • Key’s role is based on determination • A determines B (represented as A  B) • If you know the value of attribute A, you can determine the value of attribute B • Functionaldependence: • Attribute B functionally dependent on A if all rows in table that agree in value for A also agree in value for B 7 Database Systems, 8th Edition

  8. 8 Database Systems, 8th Edition

  9. Keys (continued) • Compositekey • Composed of more than one attribute • Keyattribute • Any attribute that is part of a key • If attribute B is functionally dependent on a composite key A but not on any subset of A, B is fully functionally dependent on A • Superkey • Any key that uniquely identifies each row • Candidatekey • A superkey without unnecessary attributes • A table may have several candidate keys • Primary key • A table has only one primary key, which is chosen from candidate keys 9 Database Systems, 8th Edition

  10. Keys (continued) • Nulls: • No data entry • Not permitted in primary key • Should be avoided in other attributes • Can represent • An unknown attribute value • A known, but missing, attribute value • A “not applicable” condition • Can create problems when functions such as COUNT, AVERAGE, and SUM are used • Can create logical problems when relational tables are linked 10 Database Systems, 8th Edition

  11. Keys (continued) • Controlled redundancy: • Makes the relational database work • Tables within the database share common attributes • Enables tables to be linked together • Multiple occurrences of values not redundant when required to make the relationship work • Redundancy exists only when there is unnecessaryduplication of attribute values 11 Database Systems, 8th Edition

  12. 12 Database Systems, 8th Edition

  13. A relational schema is a representation of database tables where each table is listed by its name followed by its attributes in parentheses VENDOR(VEND_CODE, VEND_CONTACT, VEND_AREACODE, VEND_PHONE) 13 Database Systems, 8th Edition

  14. Keys (continued) • Foreignkey (FK) • An attribute whose values match primary key values in the related table • Referentialintegrity • FK contains a value that refers to an existing valid tuple (row) in another relation • Secondarykey • Key used strictly for data retrieval purposes • A table may have many secondary keys 14 Database Systems, 8th Edition

  15. 15 Database Systems, 8th Edition

  16. 16 Database Systems, 8th Edition

  17. Integrity Rule Example 18 Database Systems, 8th Edition

  18. Integrity Rules • Many RDBMs enforce integrity rules automatically • Safer to ensure application design conforms to entity and referential integrity rules • Designers may use flags to avoid nulls • Flags indicate absence of some value 19 Database Systems, 8th Edition

  19. 3.4 Relational Set Operators • Relational algebra • Defines theoretical way of manipulating table contents using relational operators • Use of relational algebra operators on existing relations produces new relations. This is called the property of closure. • UNION INTERSECT • DIFFERENCE PRODUCT • SELECT PROJECT JOIN DIVIDE 20 Database Systems, 8th Edition

  20. Combines all rows from two tables, • excluding duplicate rows • Tables must have the same attribute • characteristics 21 Database Systems, 8th Edition

  21. 22 Database Systems, 8th Edition

  22. Yields all possible pairs of rows • from two tables • Also known as the Cartesian • product 23 Database Systems, 8th Edition

  23. Yields a horizontal subset of a table 24 Database Systems, 8th Edition

  24. Yields a vertical subset of a table 25 Database Systems, 8th Edition

  25. Join • Allows information to be combined from two or more tables • Real power behind the relational database, allowing the use of independent tables linked by common attributes 26 Database Systems, 8th Edition

  26. Relational Set Operators (continued) • NaturalJoin • Links tables by selecting rows with common values in common attribute(s) • Equijoin • Links tables on the basis of an equality condition that compares specified columns • Theta join • Any other comparison operator is used • Outerjoin • Matched pairs retained and any unmatched values in other table left null 27 Database Systems, 8th Edition

  27. Natural Join Step 1: Product

  28. Natural Join Step 2: Select Natural Join Step 3: Project

  29. Left Outer Join Right Outer Join 30 Database Systems, 8th Edition

  30. 31 Database Systems, 8th Edition

  31. 3.5 The Data Dictionary and System Catalog • Datadictionary • Provides detailed accounting of all tables found within the user/designer-created database • Contains (at least) all the attribute names and characteristics for each table in the system • Contains metadata: data about data • Sometimes as “the database designer’s database” because it records the design decisions about tables and their structures 32 Database Systems, 8th Edition

  32. Systemcatalog • Contains metadata • Detailed system data dictionary that describes all objects within the database • System catalog allows RDBMS to check for and eliminate homonyms and synonyms

  33. Y 100-999 FK AGENT 100-999 34 Database Systems, 8th Edition

  34. 3.6 Relationships within the Relational Database • 1:M relationship • Relational modeling ideal • Should be the norm in any relational database design • 1:1 relationship • Should be rare in any relational database design • M:N relationships • Cannot be implemented as such in the relational model • M:N relationships can be changed into two 1:M relationships 35 Database Systems, 8th Edition

  35. The 1:M Relationship • Relational database norm • Found in any database environment 36 Database Systems, 8th Edition

  36. 37 Database Systems, 8th Edition

  37. The 1:1 Relationship • One entity related to only one other entity, and vice versa • Sometimes means that entity components were not defined properly • Could indicate that two entities actually belong in the same table • Certain conditions absolutely require their use 38 Database Systems, 8th Edition

  38. 39 Database Systems, 8th Edition

  39. The M:N Relationship • Implemented by breaking it up to produce a set of 1:M relationships • Avoid problems inherent to M:N relationship by creating a compositeentity • Includes as foreign keys the primary keys of tables to be linked 40 Database Systems, 8th Edition

  40. CLASS_CODE 41 Database Systems, 8th Edition

  41. CLASS_CODE Composite Entity 42 Database Systems, 8th Edition

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  45. 3.7 Data Redundancy Revisited • Data redundancy leads to data anomalies • Such anomalies can destroy the effectiveness of the database • Foreign keys • Control data redundancies by using common attributes shared by tables • Crucial to exercising data redundancy control • Sometimes, data redundancy is necessary 46 Database Systems, 8th Edition

  46. 48 Database Systems, 8th Edition

  47. 3.8 Indexes • Orderly arrangement to logically access rows in a table • Indexkey • Index’s reference point • Points to data location identified by the key • Uniqueindex • Index in which the index key can have only one pointer value (row) associated with it • Each index is associated with only one table 49 Database Systems, 8th Edition

  48. 50 Database Systems, 8th Edition

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