: Mrs. GOONJAN JAIN
SUBMITTED BY : PANKAJ JAISWAL
An ER-model is also called an ER-diagram.It describes the
graphical representation of entities and their relationships to each other.
The ER model consists of three basic elements: entities, attributes and
Construction of E-R Diagrams
1. Identify Entities:
Modelling the entities is the first
step to create an ER data model.
An entity is an
object, event or concept which is usually identified by a single, unique
Generally, when a data model has an identifier for
a possible entity, it is likely to be the entity. For ex, if there is a field
for customer name or customer number, then the entity relationship data model should
have an entity name CUSTOMER .
Identify the attributes for each entity.
If the price of a product is constant for all orders, then PRICE is a
function of PRODUCT. If the price of the same product is different for
different orders, then the PRICE is a function of the relationship between
ORDER and PRODUCT.
ORDER_ID, ORDER_DATE, ORDER_AMOUNT
CUSTOMER_ID, CUSTOMER_NAME, CUSTOMER_ADDRESS,
PRODUCT_ID, DESCRIPTION, PRODUCT_PRICE,
However, in general all cases are not
so simple and clear. For example, take ADDRESS, maybe a number of attributes for
this candidate entity were identified, like HOUSE_NO, STREET_NO, STATE, ZIP_CODE,
COUNTRY. Does it indicate that ADDRESS is an entity? however, all of these
should be the attributes of CUSTOMER entity.
3. Choose Primary Keys:
Primary keys must
be chosen for each entity after identifying and modeling each entity and its
Primary Key should have following properties
1. Entity should be uniquely
2. Not null (it should always has a
3. Data less needed
4. Never changes it value and
Properties of a Good Primary Key
4. Identify the Relationships:
Relationships among entities are
important part of the ER (model)Diagram. They provide the joints among the
different tables that give the database its flexibility, when these
relationships are used in the database. To increase the flexibility of the
database, relationships must be properly recognized and modeled.
Most of the relationships are simple and easy to identified, there is
relationship between ORDERS and CUSTOMERS, ORDER and PRODUCTS. It is visible that there may
not be a relationship between PRODUCT and CUSTOMER but both are related to
ORDER. we can find which products are ordered by a particular customer.
There are both maximum and minimum
ONE TO ONE
ONE TO MANY
MANY TO MANY
EXACTLY ONE ONR OR MORE
ZERO OR ONE ZERO OR MORE
There are two relationships, one
between PRODUCT and CUSTOMER, and the other between ORDER and PRODUCT.
Each customer can place minimum – 0, maximum – many orders
Each order can be placed by: minimum – 1 maximum – one customers
Each order can list: minimum – 1 maximum – many products
Each product can be listed on: minimum – 0 maximum – many orders
The final step in creating an E-R
diagram is often overlooked, but is just as important as any of the previous
steps. Analysts who fail to carefully check their ERD often produce diagrams of
poor quality, which of course should be avoided.
In order to check the ERD, you must
return to your original information sources, the forms, reports, and interviews
with users. The basic idea is to go back to the original documents and make
sure that the structure represented in the ERD can satisfy the requirements.