- Entity:
- Entity
= an object in the
real world
Example:
- you are an example of
an
"entity"
- A car, boat, ....
- you are an example of
an
"entity"
Important:
- An entities
have
properties
(=
attributes ) !!!
e.g., name, age, sex, SSN, etc.
- Entity
= an object in the
real world
- Entity type:
- Entity type
= the collection (= set) of
entities
that have the
same set
of properties (= attributes) or
characteristics
Example:
- Student is an
entity type
Because:
- Every student has
the same set of properties
e.g.: a student ID, a major, a GPA, etc
- Every student has
the same set of properties
- Student is an
entity type
- Entity type
= the collection (= set) of
entities
that have the
same set
of properties (= attributes) or
characteristics
- Entity instance:
- Entity instance =
one particular
representative
of an entity type
Example:
- John is an entity instance of entity type student
- Entity instance =
one particular
representative
of an entity type
- Entity set:
- Entity set =
a collection
of entity instances
at a
specific moment
in time
Example:
- {John, James, Mary} are
students in
CS377 Fall 2017
Which is an example of an Entity set
- {John, James, Mary} are
students in
CS377 Fall 2017
- Entity set =
a collection
of entity instances
at a
specific moment
in time
- Entity type schema:
- Entity type schema
= the description of an
entity type
- A entity type schema consists
of:
- The entity identifier (name) and
- a list of properties (= attributes) of the entity type
Example:
- Student(
StuID, Name, Address, Major, GPA )
is an
entity type schema
- Student is the
entity identifier
(or the name of
the entity type)
- The list of attributes is: StuID, Name, Address, Major, GPA
- Student is the
entity identifier
(or the name of
the entity type)
- Entity type schema
= the description of an
entity type
- Summary: regarding
to the entity "student"
- Entity type =
student
- Sample of an
entity schema:
Student( StudID, Name, Address, Level, GPA )
- Sample of an
entity instance:
(12345, John Doe, 123 West Lane, Junior, 3.5)
- Sample of an
entity set:
{ (12345, John Doe, 123 West Lane, Junior, 3.5) (99999, Peter Pan, 999 Never Land, Freshman, 2.0) }
- Entity type =
student