Entities
•
Refers to entity set and not to single
entity occurrence
•
Corresponds to table and not to row in
relational environment
•
In Chen and Crow
’s Foot models,
entity represented by rectangle with
entity’s name
•
Entity name, a noun, written in capital
letters
Attributes (continued)
•
Required attribute
: must have a value
•
Optional attribute
: may be left empty
•
Domain: set of possible values for an
attribute
•
Attributes may share a domain
•
Identifiers
: one or more attributes that
uniquely identify each entity instance
•
Composite identifier
: primary key
composed of more than one attribute
Attributes (continued)
•
M:N relationships and multivalued attributes should
not be implemented
•
Create several new attributes for each of the
original multivalued attributes components
•
Create new entity composed of original
multivalued attributes components
•
Derived attribute
: value may be
calculated from other attributes
•
Need not be physically stored within
database
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 only one side of the
relationship is known
Connectivity and Cardinality
•
Connectivity
•
Describes 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