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ObjectivesObjectives Definition of termsDefinition of terms Importance of data modelingImportance of data modeling Write good names and definitions for entities, Write good names and definitions for entities,
relationships, and attributesrelationships, and attributes Distinguish unary, binary, and ternary relationshipsDistinguish unary, binary, and ternary relationships Model different types of attributes, entities, Model different types of attributes, entities,
relationships, and cardinalitiesrelationships, and cardinalities Draw E-R diagrams for common business situationsDraw E-R diagrams for common business situations Convert many-to-many relationships to associative Convert many-to-many relationships to associative
entitiesentities Model time-dependent data using time stampsModel time-dependent data using time stamps
Statements that define or constrain Statements that define or constrain some aspect of the businesssome aspect of the business
Assert business structureAssert business structure Control/influence business behaviorControl/influence business behavior Expressed in terms familiar to end Expressed in terms familiar to end
usersusers Automated through DBMS softwareAutomated through DBMS software
Declarative–what, not howDeclarative–what, not how Precise–clear, agreed-upon meaningPrecise–clear, agreed-upon meaning Atomic–one statementAtomic–one statement Consistent–internally and externallyConsistent–internally and externally Expressible–structured, natural Expressible–structured, natural
languagelanguage Distinct–non-redundantDistinct–non-redundant Business-oriented–understood by Business-oriented–understood by
Related to business, not technical, Related to business, not technical, characteristicscharacteristics
Meaningful and self-documentingMeaningful and self-documenting UniqueUnique ReadableReadable Composed of words from an approved Composed of words from an approved
Explanation of a term or factExplanation of a term or fact Term–word or phrase with specific meaningTerm–word or phrase with specific meaning Fact–association between two or more termsFact–association between two or more terms
Guidelines for good data definitionGuidelines for good data definition Gathered in conjunction with systems Gathered in conjunction with systems
requirementsrequirements Accompanied by diagramsAccompanied by diagrams Iteratively created and refinedIteratively created and refined Achieved by consensusAchieved by consensus
E-R Model ConstructsE-R Model Constructs Entities:Entities:
Entity instance–person, place, object, event, concept (often Entity instance–person, place, object, event, concept (often corresponds to a row in a table)corresponds to a row in a table)
Entity Type–collection of entities (often corresponds to a Entity Type–collection of entities (often corresponds to a table)table)
Relationships:Relationships: Relationship instance–link between entities (corresponds to Relationship instance–link between entities (corresponds to
primary key-foreign key equivalencies in related tables)primary key-foreign key equivalencies in related tables) Relationship type–category of relationship…link between Relationship type–category of relationship…link between
entity typesentity types
Attribute–Attribute–property or characteristic of an entity or property or characteristic of an entity or relationship type (often corresponds to a field in a table)relationship type (often corresponds to a field in a table)
AttributesAttributes Attribute–property or characteristic Attribute–property or characteristic
of an entity or relationahip typeof an entity or relationahip type Classifications of attributes:Classifications of attributes:
Required versus Optional AttributesRequired versus Optional Attributes Simple versus Composite AttributeSimple versus Composite Attribute Single-Valued versus Multivalued Single-Valued versus Multivalued
AttributeAttribute Stored versus Derived AttributesStored versus Derived Attributes Identifier AttributesIdentifier Attributes
Identifier (Key)–An attribute (or Identifier (Key)–An attribute (or combination of attributes) that combination of attributes) that uniquely identifies individual uniquely identifies individual instances of an entity typeinstances of an entity type
Simple versus Composite IdentifierSimple versus Composite Identifier Candidate Identifier–an attribute that Candidate Identifier–an attribute that
could be a key…satisfies the could be a key…satisfies the requirements for being an identifierrequirements for being an identifier
More on RelationshipsMore on Relationships Relationship Types vs. Relationship InstancesRelationship Types vs. Relationship Instances
The relationship type is modeled as lines between The relationship type is modeled as lines between entity types…the instance is between specific entity types…the instance is between specific entity instancesentity instances
Relationships can have attributesRelationships can have attributes These describe features pertaining to the association These describe features pertaining to the association
between the entities in the relationshipbetween the entities in the relationship
Two entities can have more than one type of Two entities can have more than one type of relationship between them (multiple relationship between them (multiple relationships)relationships)
Associative Entity–combination of Associative Entity–combination of relationship and entityrelationship and entity
Degree of a relationship is Degree of a relationship is the number of entity types the number of entity types that participate in itthat participate in itUnary RelationshipUnary RelationshipBinary RelationshipBinary RelationshipTernary RelationshipTernary Relationship
Cardinality of RelationshipsCardinality of Relationships
One-to-OneOne-to-One Each entity in the relationship will have exactly Each entity in the relationship will have exactly
one related entityone related entity One-to-ManyOne-to-Many
An entity on one side of the relationship can An entity on one side of the relationship can have many related entities, but an entity on have many related entities, but an entity on the other side will have a maximum of one the other side will have a maximum of one related entityrelated entity
Many-to-ManyMany-to-Many Entities on both sides of the relationship can Entities on both sides of the relationship can
have many related entities on the other sidehave many related entities on the other side
Cardinality ConstraintsCardinality Constraints Cardinality Constraints - the number of Cardinality Constraints - the number of
instances of one entity that can or must instances of one entity that can or must be associated with each instance of be associated with each instance of another entityanother entity
Minimum CardinalityMinimum Cardinality If zero, then optionalIf zero, then optional If one or more, then mandatoryIf one or more, then mandatory
Maximum CardinalityMaximum Cardinality The maximum numberThe maximum number
Strong vs. Weak Entities, andStrong vs. Weak Entities, andIdentifying RelationshipsIdentifying Relationships
Strong entities Strong entities exist independently of other types of entitiesexist independently of other types of entities has its own unique identifierhas its own unique identifier identifier underlined with single-lineidentifier underlined with single-line
Weak entityWeak entity dependent on a strong entity (identifying owner)…cannot exist on its dependent on a strong entity (identifying owner)…cannot exist on its
ownown does not have a unique identifier (only a partial identifier)does not have a unique identifier (only a partial identifier) Partial identifier underlined with double-linePartial identifier underlined with double-line Entity box has double lineEntity box has double line
Identifying relationshipIdentifying relationship links strong entities to weak entitieslinks strong entities to weak entities
Associative EntitiesAssociative Entities An An entityentity–has attributes–has attributes
A A relationshiprelationship–links entities together–links entities together
When should a When should a relationship with attributesrelationship with attributes instead be instead be an an associative entityassociative entity? ? All relationships for the associative entity should be manyAll relationships for the associative entity should be many The associative entity could have meaning independent of the The associative entity could have meaning independent of the
other entitiesother entities The associative entity preferably has a unique identifier, and The associative entity preferably has a unique identifier, and
should also have other attributesshould also have other attributes The associative entity may participate in other relationships The associative entity may participate in other relationships
other than the entities of the associated relationshipother than the entities of the associated relationship Ternary relationships should be converted to associative Ternary relationships should be converted to associative
Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right.
Note that the many-to-many cardinality between entities in Figure 3-11a has been replaced by two one-to-many relationships with the associative entity.