: Mrs. GOONJAN JAIN
SUBMITTED BY : PANKAJ JAISWAL
An entity relationship model is also called an entity-relationship diagram.It is a 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_NO, DATE, AMOUNT
CUSTOMER_ID, NAME, ADDRESS, NUMBER
PRODUCT_ID, DESCRIPTION, PRICE
However, all cases are not so clear.
For example, take ADDRESS, perhaps a number of attributes were identified for
this candidate entity, like HOUSE NUMBER, STREET, STATE, ZIP_CODE. Does it
indicate that ADDRESS is an entity? however, all of these should be attributes
of CUSTOMER entity.
3. Choose Primary Keys:
and modeling each entity and its attributes, primary keys must be chosen for
Desirable Primary Key
1. Uniquely identifies an entity
2. Non-null (always has a value)
4. Never changes
Characteristics of a Good Primary Key
4. Identify the Relationships:
Relationships among entities are a
critical part of the Entity Relationship Diagram. When these relationships are
implemented in the database, they provide the links among the various tables
that give the database its flexibility. To maximize the flexibility of a
database, relationships must be properly identified and modeled.
Many relationships are relatively easy to recognize, such as those
between ORDERS and CUSTOMERS, ORDER and CUSTOMER are related, as are ORDER and PRODUCT. However, we
know that there may not necessarily be a relationship between PRODUCT and
CUSTOMER. Because both are related to ORDER we can report which products are
ordered by a particular customer.
5. Determine Cardinalities:
There are both maximum and minimum
ONE TO ONE
ONE TO MANY
MANY TO MANY
EXACTLY ONEONR OR MORE
ZERO OR ONEZERO OR MORE
There are two relationships, one
between PRODUCT and ORDER, and the other between ORDER and CUSTOMER.
Each customer can place: min =0, max = many orders
Each order can be placed by: min one max one customers
Each order can list: min one max many products
Each product can be listed on: min zero max 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.