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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
Understanding Entity Relationship Diagrams
  • Notation basics
  • Understanding relationships
  • Generalization hierarchies
  • Business rule representation
  • Diagram rules
  • Alternative notations
classification of cardinalities
Classification of Cardinalities
  • Minimum cardinality based
    • Mandatory: existence dependent
    • Optional
  • Maximum cardinality based
    • Functional
    • 1-M
    • M-N
    • 1-1
understanding relationships
Understanding Relationships
  • Identification dependency
  • M-N relationships with attributes
  • Self identifying relationships
  • M-way relationships
  • Equivalence between M-N and 1-M relationships
relationship equivalence
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
inheritance
Inheritance
  • Subtypes inherit attributes of supertypes (direct and indirect)
  • Allows abbreviation of attribute list
  • Applies to code (methods) as well as attributes (data)
business rules
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
formal representation
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
informal representation
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
diagram rules
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
completeness rules
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
primary key rule issue
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
naming consistency rules
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.
relationship names
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
connection consistency rules
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.
identification dependency rules
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
support in the er assistant
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
erd variations
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
erd rule variations
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
unified modeling language
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
summary
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