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Database Systems: Design, Implementation, and Management Eighth Edition

Database Systems: Design, Implementation, and Management Eighth Edition. Chapter 4 Entity Relationship (ER) Modeling. Objectives. In this chapter, you will learn: The main characteristics of entity relationship components

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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 4 Entity Relationship (ER) Modeling

  2. Objectives • In this chapter, you will learn: • The main characteristics of entity relationship components • How relationships between entities are defined, refined, and incorporated into the database design process • How ERD components affect database design and implementation • That real-world database design often requires the reconciliation (調和) of conflicting goals Database Systems, 8th Edition

  3. 4.1 The Entity Relationship (ER) Model • ER model forms the basis of an ER diagram • ERD represents conceptual database as viewed by end user • ERDs depict database’s main components: • Entities • Attributes • Relationships Database Systems, 8th Edition

  4. Entities • Refers to entity set and not to single entity occurrence • Corresponds to table and not to row in relational environment • In Chen and Crow’s Foot models, entity represented by rectangle with entity’s name • Entity name, a noun, written in capital letters (大寫) Database Systems, 8th Edition

  5. Attributes • Characteristics (特性) of entities • Chen notation: attributes represented by ovals connected to entity rectangle with a line • Each oval contains the name of attribute it represents • Crow’s Foot notation: attributes written in attribute box below entity rectangle Database Systems, 8th Edition

  6. Database Systems, 8th Edition

  7. Attributes (continued) • Required (必要) attribute: must have a value, represented by bold font (粗體字) • Optional (選擇) attribute: may be left empty • Domain: set of possible values for an attribute • Attributes may share a domain • Identifiers(識別碼): one or more attributes that uniquely identify each entity instance • Composite (組合)identifier: primary key composed of more than one attribute Database Systems, 8th Edition

  8. Database Systems, 8th Edition

  9. Attributes (continued) • Composite attribute (組合屬性) can be subdivided (分解) • Simple attribute cannot be subdivided • Single-value attribute can have only a single value • Multi-valued attributes (多重數值屬性) can have many values Database Systems, 8th Edition

  10. Database Systems, 8th Edition

  11. Attributes (continued) • M:N relationships and multi-valued attributes should not be implemented • Create several new attributes for each of the original multi-valued attributes components • Create new entity composed of original multi-valued attributes components • Derived(衍生)attribute: value may be calculated from other attributes • Need not be physically stored within database Database Systems, 8th Edition

  12. Database Systems, 8th Edition

  13. Relationships • Association (關聯性) between entities • Participants (參與者) are entities that participate in a relationship • Relationships between entities always operate in both directions • Relationship can be classified as 1:M or 1:1 • Relationship classification is difficult to establish if only one side of the relationship is known Database Systems, 8th Edition

  14. Connectivity and Cardinality • Connectivity (連接性) • Describes the relationship classification as 1-to-1, 1-to-many, or many-to-many • Cardinality (基數) • Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity using the format (x, y) • Established by very concise statements known as business rules Database Systems, 8th Edition

  15. Database Systems, 8th Edition

  16. Existence Dependence • Existencedependence (存在相依) • Entity exists in database only when it is associated with another related entity occurrence • Existenceindependence (存在獨立) • Entity can exist apart from one or more related entities • Sometimes such an entity is referred to as a strong or regular entity Database Systems, 8th Edition

  17. Relationship Strength • Weak (non-identifying) relationships (弱關係,非確定性關係) • Exists if PK of related entity does not contain PK component of parent entity • Strong (identifying) relationships (強關係,確定性關係) • Exists when PK of related entity contains PK component of parent entity Database Systems, 8th Edition

  18. 虛線 Fig 4.8 A weak relationship between COURSE and CLASS

  19. 實線 Fig 4.9 A strong relationship between COURSE and CLASS

  20. Weak Entities • Weakentity (弱實體) meets two conditions • Existence-dependent • Primary key partially or totally derived from parent entity in relationship • Database designer determines whether an entity is weak based on business rules Database Systems, 8th Edition

  21. 實線 Fig 4.10 A weak entity in an ERD

  22. Fig 4.11 A weak entity in a strong relationship

  23. Relationship Participation • Optionalparticipation (選擇性參與) • One entity occurrence does not require corresponding entity occurrence in particular relationship • Mandatoryparticipation (強制性參與) • One entity occurrence requires corresponding entity occurrence in particular relationship Database Systems, 8th Edition

  24. Relationship Degree (關係程度) • Indicates number of entities or participants associated with a relationship • Unaryrelationship (一元關係) • Association is maintained within single entity • Binaryrelationship(二元關係) • Two entities are associated • Ternaryrelationship(三元關係) • Three entities are associated Database Systems, 8th Edition

  25. Database Systems, 8th Edition

  26. Fig 4.16 The implementation of a ternary relationship

  27. Recursive (遞迴) Relationships • Relationship can exist between occurrences of the same entity set • Naturally found within unary relationship Database Systems, 8th Edition

  28. Fig 4.20 Implementation of the M:N recursive “PART contains PART” relationship

  29. Database Systems, 8th Edition

  30. Associative (Composite) Entities (關聯實體、組合實體) • Also known as bridge entities (橋接實體) • Used to implement M:N relationships • Composed of primary keys of each of the entities to be connected • May also contain additional attributes that play no role in connective process Database Systems, 8th Edition

  31. Database Systems, 8th Edition

  32. Database Systems, 8th Edition

  33. 4.2 Developing an ER Diagram • Database design is an iterative process (反覆式流程) • Create detailed narrative of organization’s description of operations • Identify business rules based on description of operations • Identify main entities and relationships from business rules • Develop initial ERD • Identify attributes and primary keys that adequately describe entities • Revise and review ERD Database Systems, 8th Edition

  34. Database Systems, 8th Edition

  35. Database Systems, 8th Edition

  36. Database Systems, 8th Edition

  37. Database Systems, 8th Edition

  38. Database Systems, 8th Edition

  39. Database Systems, 8th Edition

  40. Fig 4.36 The conceptual UML class diagram for Tiny College

  41. Fig 4.37 The implementation-ready UML class diagram for Tiny College

  42. 4.3 Database Design Challenges: Conflicting Goals • Database designers must make design compromises • Conflicting goals: design standards, processing speed, information requirements • Important to meet logical requirements and design conventions (慣例) • Design of little value unless it delivers all specified query and reporting requirements • Some design and implementation problems do not yield “clean” solutions Database Systems, 8th Edition

  43. Fig 4.38 Various Implementation of the 1:1 recursive relationship

  44. Summary • Entity relationship (ER) model • Uses ERD to represent conceptual database as viewed by end user • ERM’s main components: • Entities • Relationships • Attributes • Includes connectivity and cardinality notations Database Systems, 8th Edition

  45. Summary (continued) • Connectivities and cardinalities are based on business rules • M:N relationship is valid at conceptual level • Must be mapped to a set of 1:M relationships • ERDs may be based on many different ERMs • Database designers are often forced to make design compromises Database Systems, 8th Edition

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