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Understanding Entity Relationship Diagrams

Understanding Entity Relationship Diagrams. Notation basics Understanding relationships Generalization hierarchies Business rule representation Diagram rules Alternative notations. Basic Symbols. Cardinalities. Cardinality Notation. Classification of Cardinalities.

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Understanding Entity Relationship Diagrams

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  1. Understanding Entity Relationship Diagrams • Notation basics • Understanding relationships • Generalization hierarchies • Business rule representation • Diagram rules • Alternative notations

  2. Basic Symbols

  3. Cardinalities

  4. Cardinality Notation

  5. Classification of Cardinalities • Minimum cardinality based • Mandatory: existence dependent • Optional • Maximum cardinality based • Functional • 1-M • M-N • 1-1

  6. Summary of Cardinalities

  7. More Relationship Examples

  8. Comparison to Access Notation

  9. Understanding Relationships • Identification dependency • M-N relationships with attributes • Self identifying relationships • M-way relationships • Equivalence between M-N and 1-M relationships

  10. Identification Dependency

  11. M-N Relationships with Attributes

  12. M-N Relationships with Attributes (II)

  13. Instance Diagrams for Self-Referencing Relationships

  14. ERD Notation for Self-Referencing Relationships

  15. Associative Entity Types for M-way Relationships

  16. Relationship Equivalence • Replace M-N relationship • Associative entity type • Two identifying 1-M relationships • M-N relationship versus associative entity type • Largely preference • Associative entity type is more flexible in some situations

  17. Associative Entity Type Example

  18. Generalization Hierarchies

  19. Inheritance • Subtypes inherit attributes of supertypes (direct and indirect) • Allows abbreviation of attribute list • Applies to code (methods) as well as attributes (data)

  20. Generalization Constraints

  21. Multiple Levels of Generalization

  22. Comprehensive Example

  23. Business Rules • Enforce organizational policies • Promote efficient communication • Formal representation in ERD • Informal representation in documentation associated with an ERD • Use rules language to formally represent in relational database after conversion

  24. Formal Representation • Primary key constraints: entity identification • Named relationships: direct connections among business entities • Identification dependency: knowledge of other entities for identification • Cardinalities: restrict number of related entities in a business situation • Generalization hierarchies: classification of business entities and organizational policies

  25. Informal Representation • Specify as documentation associated elements of an ERD • Candidate key constraints: alternate ways to identify business entities • Reasonable values: fixed collection of values or consistent with another attribute • Null value constraints: data collection completeness • Default values: simplify data entry and provide value when unknown

  26. Diagram Rules • Ensure that ERD notation is correctly used • Similar to syntax rules for a computer language • Completeness rules: no missing specifications • Consistency rules: no conflicts among specifications • Supported by the ER Assistant

  27. Completeness Rules • Primary Key Rule: all entity types have a PK (direct, indirect, or inherited) • Naming Rule: all entity types, relationships, and attributes have a name • Cardinality Rule: cardinality is specified in both directions for each relationship • Entity Participation Rule: all entity types participate in an at least one relationship except for entity types in a generalization hierarchy • Generalization Hierarchy Participation Rule: at least one entity type in a generalization hierarchy participates in a relationship

  28. Primary Key Rule Issue • Primary key rule is simple in most cases • For some weak entities, the PK rule is subtle • Weak entity with only one 1-M identifying relationship • Weak entity must have a local key to augment the borrowed PK from the parent entity type • Violation of PK rule if local key is missing

  29. PK Rule Violation Example

  30. Naming Consistency Rules • Entity Name Rule: entity type names must be unique • Attribute Name Rule: attribute names must be unique within each entity type and relationship • Inherited Attribute Rule: attribute names in a subtype do not match inherited (direct or indirect) attribute names.

  31. Relationship Names • No uniqueness requirement • Participating entities provide a context for relationship names • Use unique names as much as possible to distinguish relationships • Must provide unique names for multiple relationships between the same entity types

  32. Connection Consistency Rules • Relationship/Entity Connection Rule: relationships connect two entity types (not necessarily distinct) • Relationship/Relationship Connection Rule: relationships are not connected to other relationships • Redundant Foreign Key Rule: foreign keys are not used.

  33. Identification Dependency Rules • Weak entity rule: weak entities have at least one identifying relationship • Identifying relationship rule: at least one participating entity type must be weak for each identifying relationship • Identification dependency cardinality rule: the minimum and maximum cardinality must equal 1 for a weak entity in all identifying relationships

  34. Example of Diagram Errors

  35. Corrected ERD

  36. Support in the ER Assistant • Relationship formation rules are supported by diagram construction • Other rules are supported by the Check Diagram feature • For the Redundant Foreign Key rule, the ER Assistant detects FKs that have the same name as the associated PKs

  37. ERD Variations • No standard ERD notation • Symbol variations • Placement of cardinality symbols • Rule variations • Be prepared to adjust to the ERD notation in use by each employer

  38. ERD Rule Variations • Lack of ERD standards • M-way relationships • M-N relationships • Relationships with attributes • Self-referencing relationships • Relationships connected to other relationships • Adapt to notations in work environments

  39. Chen ERD Notation

  40. Unified Modeling Language • Standard notation for object-oriented modeling • Objects • Object features • Interactions among objects • UML supports class diagrams, interface diagrams, and interaction diagrams • More complex than ERD notation

  41. Simple Class Diagram

  42. Association Class

  43. Generalization Relationship

  44. Composition Relationship

  45. Summary • Data modeling is an important skill • Crow’s Foot ERD notation is widely used • Use notation precisely • Use the diagram rules to ensure structural consistency and completeness • Understanding the ERD notation is a prerequisite to applying the notation on business problems

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