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Chapter 4

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

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Chapter 4

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  1. Chapter 4 Entity Relationship (ER) Modeling Database Systems: Design, Implementation, and Management, Seventh Edition, Rob and Coronel

  2. In this chapter, you will learn: • The main characteristics of entity relationship components • How relationships between entities are defined and refined and how those relationships are incorporated into the database design process • How ERD components affect database design and implementation • That real-world database design often requires the reconciliation of conflicting goals Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  3. 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: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  4. Entities • Refers to entity set and not to single entity occurrence • Corresponds to table and not to row in relational environment • In both Chen and Crow’s Foot models, entity is represented by rectangle containing entity’s name • Entity name, a noun, is usually written in capital letters Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  5. Attributes • Characteristics of entities • In Chen model, attributes are represented by ovals and are connected to entity rectangle with a line • Each oval contains the name of attribute it represents • In Crow’s Foot model, attributes are written in attribute box below entity rectangle Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  6. Attributes (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  7. Domains • Attributes have domain • Domain is attribute’s set of possible values • Attributes may share a domain Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  8. Identifiers (Primary Keys) • Underlined in the ERD • Key attributes are also underlined in frequently used table structure shorthand Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  9. Composite Primary Keys • Primary keys ideally composed of only single attribute • Possible to use a composite key • Primary key composed of more than one attribute Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  10. Composite Primary Keys (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  11. Composite and Simple Attributes • Composite attribute can be subdivided • Simple attribute cannot be subdivided Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  12. Single-Valued Attributes • Single-value attribute can have only a single value Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  13. Multivalued Attributes • Multivalued attributes can have many values Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  14. Multivalued Attributes (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  15. Resolving Multivalued Attribute Problems • Although conceptual model can handle M:N relationships and multivalued attributes, you should not implement them in relational DBMS • Within original entity, create several new attributes, one for each of the original multivalued attribute’s components • Can lead to major structural problems in table • Create new entity composed of original multivalued attribute’s components Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  16. Resolving Multivalued Attribute Problems (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  17. Resolving Multivalued Attribute Problems (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  18. Resolving Multivalued Attribute Problems (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  19. Derived Attributes • Attribute whose value may be calculated (derived) from other attributes • Need not be physically stored within database • Can be derived by using an algorithm Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  20. Derived Attributes (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  21. Derived Attributes (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  22. Relationships • Association between entities • Participants are entities that participate in a relationship • Relationships between entities always operate in both directions • Relationship can be classified as 1:M • Relationship classification is difficult to establish if know only one side of the relationship Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  23. Connectivity and Cardinality • Connectivity • Used to describe the relationship classification • Cardinality • Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity • Established by very concise statements known as business rules Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  24. Connectivity and Cardinality (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  25. Existence Dependence • Existence dependence • Exist in database only when it is associated with another related entity occurrence • Existence independence • Entity can exist apart from one or more related entities • Sometimes refers to such an entity as strong or regular entity Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  26. Relationship Strength • Weak (non-identifying) relationships • Exists if PK of related entity does not contain PK component of parent entity • Strong (Identifying) Relationships • Exists when PK of related entity contains PK component of parent entity Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  27. Weak (Non-Identifying) Relationships Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  28. Weak (Non-Identifying) Relationships (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  29. Strong (Identifying) Relationships Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  30. Weak Entities • Weak entity meets two conditions • Existence-dependent • Cannot exist without entity with which it has a relationship • Has primary key that is partially or totally derived from parent entity in relationship • Database designer usually determines whether an entity can be described as weak based on business rules Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  31. Weak Entities (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  32. Weak Entities (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  33. Relationship Participation • Optional participation • One entity occurrence does not require corresponding entity occurrence in particular relationship • Mandatory participation • One entity occurrence requires corresponding entity occurrence in particular relationship Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  34. Relationship Participation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  35. Relationship Participation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  36. Relationship Participation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

  37. Relationship Participation (continued) Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

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