ER
diagramming is a technique that is widely used in the world of business
and information
technology to show how information is stored within a business system.
ER
diagram is a specialized graphic that defines the interrelationship
between entities
used in
computing in regard to the organizaiton of data within database or
information
system.
It is also called an entity relationship model. Is is an abstract and
conceptual
representation of data, and database modeling method, used to produce a
type of
conceptual schema or semantic data model of a system.
Data
modeling is a technique aimed at optimizing the way that information is
stored
and used
within an organization. It begins with the identification of the main
data
groups,
for example the invoice, and continues by defining the detailed content
of each
of these groups. This results in structured definitions for all of the
information
that is
stored and used within a given system. The technique provides a solide
foundation
for
system design and a universal standard for system documentaiton.
Basic components of ER-Diagram are as follows:Entity
An
entity is a piece of data-an object or concept about which data is
stored. It
is an
abstraction from the complexities of some domain. It can be applied to
almost
anything
that is significant to the system being studied. For example, Customer,
Loan,
Student, Airport etc. It is represented by a box that contain the name
of
that
entity. It may be a physical object such as a house or a car, an event
such
as a
house sale or a car service, or a concept such as a customer transaction
or
order.
An entity-type is a category. An entity, is an instance of a given entity-type. There are usually many instances if an entity-type.
Attribute
It is a element of the data group of an entity. Each entity has a data group associated with it. For example it the Car is an entity, then Car Number, Car Model, Car color etc are the attributes of Car entity. Attributes are represented by oval or circles that contains the attribute name inside it. A given attribute belonging to a given entity occurrence can only have one value.
Relationship
It captures how two or more entities are related to one another. For example an owns relationship between a company and a computer. It is represented by diamond shape symbol.
Entity Keys
Each
entity occurrence can be uniquely identified, by using an attribute or a
combination
of
attributes that is called entity key.he primary key is the attribute (or
group
of
attributes) that serve to uniquely identify each entity occurrence.
Solid
lines are used to connect these symbols to make a complete ER diagram.Relationship Cardinality
It is
the degree or maximum cardinality of the relationship. There are many
occurrences
of the
entity may relate to the other entity. There are three type of
relationship
between
entities:
- One-to-one :One
instance of an entity is associated
with
one other instance of another entity. For example, in a database of
employees,
each
employee name is associated with only one social security number.
- One-to-many :One
instance of an entity is associated
with
zero, one or many instances of another entity, but for one instance of
other
entity there is only one instance of entity one. For example, for a
company with
all
employees working in one building, the building name is associated with
many
different employees, but those employees all share the same singular
association
with
entity.
- Many-to-many :One
instance of an entity is associated
with
one, zero or many instances of another entity and same as vise versa.
For example,
for a
company in which all of its employees work on multiple projects, each
instance
of
an employee is associated with many instances of a project, and at the
same time,
each
instance of a project has multiple employees associated with it.
Following are the basic steps to start an ER diagram:
Define entites : these are usually nouns used in descriptions of the system, in the discussion of business rules, or in documentation;
Define Relationships: these are usually verbs used in descriptions of the system or in discussion of the business rules;
Add attributes to the relations: these are determined by the queries,and may also suggest new entities, e.g. grade; or they may suggest the need for keys or identifiers.
Add cardinality to the relations: Many-to-Many must be resolved to two one-to-manys with an additional entity
Define entites : these are usually nouns used in descriptions of the system, in the discussion of business rules, or in documentation;
Define Relationships: these are usually verbs used in descriptions of the system or in discussion of the business rules;
Add attributes to the relations: these are determined by the queries,and may also suggest new entities, e.g. grade; or they may suggest the need for keys or identifiers.
Add cardinality to the relations: Many-to-Many must be resolved to two one-to-manys with an additional entity
Thanks for the info! For more information on ER Diagram Symbols and Meaning (Entity Relationship Diagram), I would recommend checking out Lucidchart. Their site is very helpful and easy to use! Check it out!
ReplyDelete