Database Modeling and Design

234 Glossary
entity instance (or occurrence)—A particular occurrence of an
entity. For example, an instance of the entity actor would be
Johnny Depp.
entity-relationship (ER) diagram—A diagram (or graph) of entities
and their relationships, and possibly the attributes of those entities.
entity-relationship (ER) model—A conceptual data model involving
entities, relationships among entities, and attributes of those
entities.
exclusion constraint—A symbol (+) between two relationships in the
ER model with a common entity that implies that either one relationship
must hold at a given point in time, or the other must
hold, but not both.
existence dependency—A dependency between two entities such that
one is dependent upon the other for its existence, and cannot
exist alone. For example, an employee work-history entity cannot
exist without the corresponding employee entity. Also refers to
the connectivity between two entities as being mandatory or
optional.
fact table—The dominating table in a data warehouse and its star
schema, containing dimension attributes and data measures at the
individual data level.
fifth normal form (5NF)—A table is in fifth normal form (5NF) if and
only if there are no possible lossless decompositions into any subset
of tables; in other words, if there is no possible lossless decomposition,
then the table is in 5NF (see Section 6.5).
file—A collection of records of the same type. For example, an
employee file is a collection of employee records.
first normal form (1NF)—A table is in first normal form (1NF) if and
only if there are no repeating columns of data taken from the
same domain and having the same meaning.
foreign key—Any attribute in a SQL table (key or nonkey) that is taken
from the same domain of values as the primary key in another
SQL table and can be used to join the two tables (without loss of
data integrity) as part of a SQL query.
fourth normal form (4NF)—A table is in fourth normal form (4NF) if
and only if it is at least in BCNF and if whenever there exists a
nontrivial multivalued dependency of the form X->>Y, then X
must be a superkey in the table.