Entity Relational Model Flashcards
(11 cards)
Entity
Object of interest to the end user (effectively a table)
Entity Instance / Occurance
A row - or a tuple
Attribute
Characteristics of an entity (a field)
Composite Attribute
Attributes that can be subdivided into additional attributes (i.e address into number, street, postcode, state)
Multivalued Attributes
Attributes that can have many values (e.g. 1 person, many degrees)
Derived Attributes
An attribute whose value can be directly derived from other attributes
doesn’t need to be stored as an attribute, can be calculated on the fly
Domain
Set of possible values that an attribute can have
different attributes can share the same domain - e.g. the professor and student address will likely share the same domain
Identifiers (+ important tip with PK)
Attributes that can be used to uniquely identify an entity instance
(important - in conceptual designs you never refer to them as a primary key, this is an implementation detail)
Composite Identifier
Identifiers that are made up of two or more attributes - so it requires a unique combination of these elements
Degree
Number of attributes in an entity
Cardinality (in an entity)
Number of rows in a table