Database design, implementation, and management -chapter04

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Database design, implementation, and management -chapter04

  1. 1. 4 Chapter 4 Entity Relationship (ER) Modeling Database Systems:Design, Implementation, and Management, Sixth Edition, Rob and Coronel 1
  2. 2. 4 In this chapter, you will learn:•How relationships between entities are defined and refined, and how such relationships are incorporated into the database design process•How ERD components affect database design and implementation•How to interpret the modeling symbols for the four most popular ER modeling tools•That real-world database design often requires that you reconcile conflicting goals Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 2
  3. 3. 4 The Entity Relationship (ER) Model•ER model forms the basis of an ER diagram•ERD represents the conceptual database as viewed by end user•ERDs depict the ER model’three main s components: –Entities –Attributes –Relationships Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 3
  4. 4. 4 Entities•Refers to the entity set and not to a single entity occurrence•Corresponds to a table and not to a row in the relational environment•In both the Chen and Crow’Foot models, an s entity is represented by a rectangle containing the entity’name s•Entity name, a noun, is usually written in capital letters Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 4
  5. 5. 4 Attributes•Characteristics of entities•In Chen model, attributes are represented by ovals and are connected to the entity rectangle with a line•Each oval contains the name of the attribute it represents•In the Crow’Foot model, the attributes are s simply written in the attribute box below the entity rectangle Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 5
  6. 6. 4The Attributes of the STUDENT Entity Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 6
  7. 7. 4 Domains•Attributes have a domain: –The attribute’set of possible values s•Attributes may share a domain Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 7
  8. 8. 4 Primary Keys•Underlined in the ER diagram•Key attributes are also underlined in frequently used table structure shorthand•Ideally composed of only a single attribute•Possible to use a composite key: –Primary key composed of more than one attribute Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 8
  9. 9. 4 The CLASS Table (Entity) Components and ContentsDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 9
  10. 10. 4 Attributes•Composite attribute•Simple attribute•Single-value attribute•Multivalued attributes Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 10
  11. 11. 4 A Multivalued Attribute in an EntityDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 11
  12. 12. 4Resolving Multivalued Attribute Problems•Although the conceptual model can handle multivalued attributes, you should not implement them in the relational DBMS –Within original entity, create several new attributes, one for each of the original multivalued attribute’components s •Can lead to major structural problems in the table –Create a new entity composed of original multivalued attribute’components s Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 12
  13. 13. 4 Splitting the Multivalued Attribute into New AttributesDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 13
  14. 14. 4Components of the Multivalued Attribute Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 14
  15. 15. 4 A New Entity Set Composed of aMultivalued Attribute’Components sDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 15
  16. 16. 4 Derived Attributes•Attribute whose value may be calculated (derived) from other attributes•Need not be physically stored within the database•Can be derived by using an algorithm Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 16
  17. 17. 4 Depiction of a Derived AttributeDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 17
  18. 18. 4 Relationships•Association between entities•Participants: –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 you only know one side Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 18
  19. 19. 4 Connectivity and Cardinality•Connectivity –Used to describe the relationship classification•Cardinality –Expresses the specific number of entity occurrences associated with one occurrence of the related entity•Established by very concise statements known as business rules Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 19
  20. 20. 4Connectivity and Cardinality in an ERD Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 20
  21. 21. 4 RELATIONSHIP Strength•Existence dependence –Entity’existence depends on the existence of s one or more other entities•Existence independence –Entity can exist apart from one or more related entities•Weak (non-identifying) relationships –One entity is not existence-independent on another entity•Strong (Identifying) Relationships –Related entities are existence-dependent Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 21
  22. 22. 4A Weak (Non-Identifying) Relationship Between COURSE and CLASS Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 22
  23. 23. 4 A Weak Relationship Between COURSE and CLASSDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 23
  24. 24. 4 Relationship Participation•Optional: –One entity occurrence does not require a corresponding entity occurrence in a particular relationship•Mandatory: –One entity occurrence requires a corresponding entity occurrence in a particular relationship Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 24
  25. 25. 4 A Strong (Identifying) Relationship Between COURSE and CLASSDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 25
  26. 26. 4An Optional CLASS Entity in the Relationship PROFESSOR teaches CLASS Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 26
  27. 27. 4 COURSE and CLASS in a Mandatory RelationshipDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 27
  28. 28. 4Relationship Strength and 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 the parent entity in the relationship•Database designer usually determines whether an entity can be described as weak based on the business rules Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 28
  29. 29. 4 A Weak Entity in an ERDDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 29
  30. 30. 4A Weak Entity in a Strong Relationship Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 30
  31. 31. 4 Relationship Degree•Indicates number of associated entities or participants•Unary relationship –Association is maintained within a single entity•Binary relationship –Two entities are associated•Ternary relationship –Three entities are associated Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 31
  32. 32. 4 Three Types of RelationshipsDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 32
  33. 33. 4 The Implementation of a Ternary RelationshipDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 33
  34. 34. 4 Recursive Relationships•Relationship can exist between occurrences of the same entity set•Naturally found within a unary relationship Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 34
  35. 35. 4 An ER Representation of Recursive RelationshipsDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 35
  36. 36. 4 The 1:1 Recursive Relationship“EMPLOYEE is Married to EMPLOYEE” Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 36
  37. 37. 4Implementation of the M:N Recursive“PART Contains PART” RelationshipDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 37
  38. 38. 4 Implementation of the 1:M “ EMPLOYEEManages EMPLOYEE” Recursive Relationship Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 38
  39. 39. 4 Composite Entities•Also known as bridge entities•Composed of the primary keys of each of the entities to be connected•May also contain additional attributes that play no role in the connective process Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 39
  40. 40. 4 Converting the M:N Relationship into Two 1:M RelationshipsDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 40
  41. 41. 4 The M:N Relationship Between STUDENT and CLASSDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 41
  42. 42. 4 A Composite Entity in an ERDDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 42
  43. 43. 4 Entity Supertypes and Subtypes•Generalization hierarchy –Depicts a relationship between a higher-level supertype entity and a lower-level subtype entity•Supertype entity –Contains shared attributes•Subtype entity –Contains unique attributes Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 43
  44. 44. 4 Nulls Created by Unique AttributesDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 44
  45. 45. 4 A Generalization HierarchyDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 45
  46. 46. 4 Disjoint Subtypes•Also known as non-overlapping subtypes –Subtypes that contain a subset of the supertype entity set –Each entity instance (row) of the supertype can appear in only one of the disjoint subtypes•Supertype and its subtype(s) maintain a 1:1 relationship Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 46
  47. 47. 4 The EMPLOYEE/PILOT Supertype/Subtype RelationshipDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 47
  48. 48. 4 A Generalization Hierarchy with Overlapping SubtypesDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 48
  49. 49. 4A Comparison of ER Modeling Symbols Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 49
  50. 50. 4 The Chen Representation of the Invoicing ProblemDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 50
  51. 51. 4 The Crow’Foot Representation s of the Invoicing ProblemDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 51
  52. 52. 4 The Rein85 Representation of the Invoicing ProblemDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 52
  53. 53. 4 The IDEF1X Representation of the Invoicing ProblemDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 53
  54. 54. 4 Developing an ER Diagram•Database design is an iterative rather than a linear or sequential process•Iterative process –Based on repetition of processes and procedures Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 54
  55. 55. 4 A Supertype/Subtype RelationshipDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 55
  56. 56. 4 A Supertype/Subtype Relationship in an ERDDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 56
  57. 57. 4 Components of the ER ModelDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 57
  58. 58. 4 The Completed Tiny College ERDDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 58
  59. 59. 4 The Challenge of Database Design: Conflicting Goals•Database design must conform to design standards•High processing speeds are often a top priority in database design•Quest for timely information might be the focus of database design Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 59
  60. 60. 4 Various Implementations of a 1:1 Recursive RelationshipDatabase Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 60
  61. 61. 4 Summary•Entity relationship (ER) model –Uses ER diagrams to represent conceptual database as viewed by the end user –Three main components •Entities •Relationships •Attributes –Includes connectivity and cardinality notations•Connectivities and cardinalities are based on business rules Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 61
  62. 62. 4 Summary (continued)•ER symbols are used to graphically depict the ER model’components and relationships s•ERDs may be based on many different ER models•Entities can also be classified as supertypes and subtypes within a generalization hierarchy•Database designers are often forced to make design compromises Database Systems: Design, Implementation, & Management, 6th Edition, Rob & Coronel 62

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