Entity, attribute, attribute value, relationship instance, composite
attribute, multivalued attribute, derived attribute, complex
attribute,.......
Entity: An entity is a thing in the real world. It may be an object with
a physical existence or an object with a conceptual existence. For
example, a man, a building, a company, a school etc.
Attribute:
Attribute is a property of an object or an entity. For example, a car
has a color, a brand name, a model number, an owner’s name, type etc.
Attribute
value: Attribute value is the real data of a particular entity for each
of its attributes. In other word, associated with each real world
entities are certain attributes that describe that entity; value of
these attributes for any entity is called attribute value. For example,
attribute value of first_name of attribute of student_name can be
Gyanendra.
Relationship instance: Each relationship instance ri
in R is an association of entities, where the association includes
exactly one entity from each participating entity type. Each such
relationship instance ri represent the fact that the entities
participating in ri are related in some way in the corresponding
miniworld situation. For example, in relationship type WORKS_FOR
associates one EMPLOYEE and DEPARTMENT, which associates each employee
with the department for which the employee works. Each relationship
instance in the relationship set WORKS_FOR associates one EMPLOYEE and
one DEPARTMENT.
Composite attribute: Composite attribute is an
attribute that can be divided into smaller subparts, which represent
more basic attributes with dependent meanings, is called a composite
attribute. For example, the Address attribute consists of several
domains such as house number, street number, city, country, etc.
Multivalued
attribute: A multi-valued attribute can have more than one value at one
time. For example, address of a person is a multi-valued attribute
since a person can have more than one address such as Present and
Permanent address. Upper and lower bounds may be placed on the number of
values in a multi-valued attribute. For example, a bank may limit the
number of addresses recorded for a single customer to two.
Derived
attribute: If an attribute’s value can be determined from the values of
other attributes, then the attribute is derivable, and is said to be a
derived attribute. Example: consider attributes for an employee: birth
date, current age; here, age is derivable by subtracting the birth date
from the current date.
Complex attribute: Composite and
multivalued attribute can be nested arbitrarily. Arbitrary nesting can
be represented by grouping components of a composite attribute between
parenthesis and separating the components with commas and by displaying
multivalued attributes between braces. Such attributes are called
composite attributes. For example, if a person has more than one address
and each residence has multiple phones and address_phone attribute can
be specifies as:
{AddressPhone( (Phone{AreaCode,PhoneNumber)},Address(StreetAddress(Number, Street, ApartmentNumber), City, State, Zip) ) }
Key
attribute: Each real world entity is unique in itself. There are
certain attributes whose value is different for all similar type of
entities. Those attributes are called key attributes. These attributes
are used to specify uniqueness constraint in relation. For example, a
house has a registration number. This is a key of all entity of house.
Value
set (domain): There is a range of values from which a particular
attriute can take value for a attribute of a real world entity. For
example, salary attribute of an employee must have value, let, from
$2000 to $12000, and then all integers in range $2000 to $12000 are
domain of attribute salary.
