Database Reference
In-Depth Information
In E-R approach approach, a given universe of discourse is represented using an entity
model: a model built up of entities, relationships and attributes. The most abstract level of
a database design is the data model, the conceptual description of a problem space. Data
models are expressed in terms of entities, attributes, domains, and relationships.
Relational database systems have the following characteristics: All data is conceptually
represented as an orderly arrangement of data into rows
and columns, called a relation.
All values are scalar (A single, non-repeating value). That is, at any given row/
column position in the relation there is one and only one value. All operations are per-
formed on an entire relation and result in an entire relation,
a concept known as closure
3.2 ENTITIES
Definition :- An entity type is a class of entity occurrences characterized by the same at-
tributes.
An entity is anything about which the system needs to store information. They can be phys-
ical (e.g., individuals, products, or buildings) or logical (e.g., departments, accounts, or
ideas). We have to capture data about entities and then store data about them in a database.
An entity is represented as a rectangle on the diagram. An entity is a physical or abstract
object that exists and can be distinguished from other objects.
For example, the Student entity in following figure represents the collection of all students
of a college/university. The individual student is called instance of the entity. Each entity
in the entity class is represented by a row of data often referred to as a record.
Example 1
Ram Pal with a student registration number 006226124023 is an entity since the distinct
features described by (registration-number, name, address, city) uniquely identify a partic-
ular person existing in the universe.
3.2.1 Entity Types
An entity type is a class of entity occurrences characterized by the same attributes.
Some entities can be broken down into more specific categories or types. The more
detailed entities are called subtypes. The more general entity to which they belong is
called a super type. The super type is called a super class and the subtypes are called
subclasses of the super class. It is essential to understand that subtypes break down
entities by type rather than by state, meaning their mode or condition. An easy way to
distinguish the two is that existing entities can change state, but they seldom, if ever,
change type.
