The Entity Relationship Model
As discussed earlier, the first step in the database design process is to identify the
organisations data requirements. Once the data requirements have been identified,
the next step is to model the data as a conceptual
schema for the database. The Entity
Relationship Model (ERM) was first described by Chen in 1976,
and is widely accepted as the standard for conceptual data modelling in the relational
database environment. The ERM is a detailed,
representation of the entities, associations and data elements for an organisation.
Because conceptual models are only concerned with what
represented in the database, the ERM does not include components to represent how the
model is implemented.
The Entity Relationship Diagram (ERD)
is a graphical representation of an ERM and is currently one of the most popular and
widely used database design tools.
of the ER Model
The three main components of the ER Model are entities,
attributes and relationships.
|In ERM terms, an entity
is a "thing" within the organisation, that we want to keep information about,
such as a customer, employee or course. In other words, an entity in an ERM actually
refers to a table, and rows within the table are referred to as entity
Entities are represented by
rectangles containing the name of the entity. Entity names must be singular and in
|Each entity has
attributes which are the properties of
each entity. Attributes will be implemented as columns in the tables. Each
attribute has a domain which
specifies the set of possible values an attribute can have. For instance, the range
of values for a telephone extension may be specified as a set of integer numbers between
4000 and 4999. An attributes domain is not displayed in ER diagrams, but is recorded
in the data dictionary.
Attributes are represented by ovals with lines connecting
them to the entity.
|Attributes can be
of various types. A composite attribute
can be subdivided into smaller parts. For example, an attribute Name can be
subdivided into First Name and Last Name. Attributes that cannot be subdivided are
called simple attributes.
First Name and Last Name are now simple attributes.
Most attributes have only a single value and as such are called
single valued attributes. For
example, a Teacher can have only one Last Name or a Subject can have only one Subject
Code. Multivalued attributes
can have more than one value. For example, a Student could have more than one
Certificate or a Department may have several Extensions.
There are a number of diagramming conventions
used to represent multivalued attributes. Some conventions implement the multivalued
attribute as an oval with a double line around it. Others use a double bar to
connect the attribute to the entity. Use either method in your own diagramming.
|A key attribute
is an attribute that has a unique value for each entity occurrence. In other words,
a key attribute is used to identify each row uniquely. For example, a Subject Code
will uniquely identify each subject as no two subjects can have the same Subject Code.
attributes are represented by underlining its name.
relationship is the association between
entities or entity occurrences. For example, a Teacher teaches
Subjects or a Subject has
Offerings. You will look at relationships in more detail in the next section
[ER Model Relationships].
Relationships are represented by diamonds with straight
lines connecting the participating entities.
for ER Components