The Entity-Relationship Model
Key Terms
alternate key: all candidate keys not chosen as the primary key
candidate key: a simple or composite key that is unique (no two rows in a table may have the same value) and minimal (every column is necessary)
characteristic entities: entities that provide more information about another table
composite attributes: attributes that consist of a hierarchy of attributes
composite key: composed of two or more attributes, but it must be minimal
dependent entities: these entities depend on other tables for their meaning
derived attributes: attributes that contain values calculated from other attributes
derived entities: see dependent entities
EID: employee identification (ID)
entity: a thing or object in the real world with an independent existence that can be differentiated from other objects
entity relationship (ER) data model: also called an ER schema, are represented by ER diagrams. These are well suited to data modelling for use with databases.
entity relationship schema: see entity relationship data model
entity set:a collection of entities of an entity type at a point of time
entity type: a collection of similar entities
foreign key (FK): an attribute in a table that references the primary key in another table OR it can be null
independent entity: as the building blocks of a database, these entities are what other tables are based on
kernel: see independent entity
key: an attribute or group of attributes whose values can be used to uniquely identify an individual entity in an entity set
multivalued attributes: attributes that have a set of values for each entity
n-ary: multiple tables in a relationship
null: a special symbol, independent of data type, which means either unknown or inapplicable; it does not mean zero or blank
recursive relationship: see unary relationship
relationships: the associations or interactions between entities; used to connect related information between tables
relationship strength: based on how the primary key of a related entity is defined
secondary key an attribute used strictly for retrieval purposes
simple attributes: drawn from the atomic value domains
SIN: social insurance number
single-valued attributes: see simple attributes
stored attribute: saved physically to the database
ternary relationship: a relationship type that involves many to many relationships between three tables.
unary relationship: one in which a relationship exists between occurrences of the same entity set.