DBMS the relationship between ORDER and PRODUCT. Entity

 DBMS ASSIGNMENT

 

 

                           

 

 

                                                                                           

                                                       
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ENTITY
RELATIONSHIP  MODEL

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
relationships.

 

CUSTOMER-ORDER-PRODUCT
ER-MODEL

 

 

 

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
attribute.

 

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 .

Entity

ORDER

CUSTOMER

PRODUCT

 

2.    
Identify attributes:

 

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.

 

Entity

Attributes

ORDER

ORDER_ID, ORDER_DATE, ORDER_AMOUNT

CUSTOMER

CUSTOMER_ID, CUSTOMER_NAME, CUSTOMER_ADDRESS,
NUMBER

PRODUCT

PRODUCT_ID, DESCRIPTION, PRODUCT_PRICE,
PRODUCT_QUANTITY

 

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
attributes.

 

Primary Key should have following properties

1. Entity should be uniquely
identified.

2. Not null (it should always has a
value)

3. Data less needed

4. Never changes it value and
properties

 

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.

 

 

 

 

 

5.      Determine
Cardinalities:

 

There are both maximum and minimum
cardinalities.

 

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

 

 

6.      Model
Checking:

 

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.