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