Database Reference
In-Depth Information
For example, Student or Teacher, every member (superentity) is described by
attributes common to both types (ID, name, address) while differentiating attributes are
applicable to one type. In this case, Teacher and Student are two sub-entities with different
attributes. Where as College_Member is a Super_Entity having common fields.
3.2.2 Sets of entities
Row or records associated with an entity in an entity class is called "sets of entities".
An entity set is set of entities of the same type. It share the same properties or attributes
for example the set of all students those are studying in a college, are called entity set of
student.
Some examples of entities include:
A person entity can be employee or student.
A concept entity can be account or department.
A place entity can be state or country.
An object entity can be building or product.
3.3 ATTRIBUTES
An entity is characterised by a number of properties or attributes. Values assigned
to attributes are used to distinguish one entity from another. Following figure identifies
the attributes of an entity in the entity class Individual. The attribute Rollno is a primary
key, as indicated by underline of that attribute. a primary key is the unique attribute that
distinguishes one record in the entity type from another. We discuss primary keys in
details when discussing constraints. For every individual record or entity in the entity class,
the college/university may want to remember their Rollno, Name, Date of Birth and Ad-
dress. Attributes can be stored as single or multiple columns in a table.
3.3.1 Simple attribute
Simple or atomic attributes cannot be further divided or subdivided, hence the notion
"atomic." For example rollno is a simple attribute.
3.3.2 Composite Attribute
A composite attribute, also called a group attribute, is an attribute formed by combining
or aggregating related attributes. The naming of composite attributes should be descriptive
and general. Most data processing applications divide the name into component parts. In
that case Name, is called a composite attribute or an aggregate. Name is usually composed
of a first name, a middle name and a last name. These are called sub-attributes. The sub-at-
tributes, such as first name, middle name, and last name, are called simple, atomic, attrib-
utes.
