Tools and Techniques for Requirements Engineering and Management - Requirements Engineering and Management for Software Development Projects - page 175

1. An item is to be purchased in an organization. So a purchase order is raised on

a vendor.

2. The vendor supplies the item to the inventory.

3. The vendor raises an invoice on the organization for payment.

4. The organization makes the payment.

From this transaction we could establish relationships:

1. There is a relationship between the purchase order and the vendor

2. There is a relationship between the vendor and the inventory

3. There is a relationship between the vendor and the invoice

4. There is a relationship between the invoice and the payments

Modeling relationships of this type is referred to as data modeling.
An entity is a place, person or a thing and is described by its attributes. For
example, an employee in a payroll system is an entity. A purchase order is an
entity in a material management system. ER Diagrams (ERD) pictorially represent
the relationships between various data entities in the system.
The entity is represented by a rectangular box in the ERDs. The relationship
between entities is represented by a line in ERDs. The ends of the line represent
the type of relationship between entities. There are three types of relationship:
1.

One-to-one relationship—An item would be in one purchase order. This is
represented in the ERDs by a straight line with normal end.

2. One-to-many relationship—A purchase order could contain many items. This

is represented in the ERDs by a straight line with crow's feet at the end of the
line having ''many'' relationship.

3. Many-to-many relationship—A vendor can receive multiple purchase orders

and supply multiple materials. This is represented in the ERDs by a straight
line with crow's feet at both ends of the line.

The symbols used in ERDs and the relationships are shown in Fig. 13.2 .
Figure 13.3 depicts a simple ERD, modeling the purchasing transaction detailed
above. A purchase order entity is placed on the vendor and the vendor entity may
receive multiple purchase orders as depicted in the figure. The vendor would
supply items to warehouse inventory. The vendor could be supplying multiple
items to the warehouse against multiple purchase orders. The vendor would raise
invoices for materials supplied. The warehouse would be requesting purchase of
items which have fallen below the reordering level set for the items.
Of course, real life ERDs would be much more elaborate and complex as there
would be many entities in a system and they would have complex relationships
with each other. The real life ERDs would span across many sheets. Many soft-
ware tools are available for modeling entity relationships and some of them could
be using different types of notations, especially in representing the relationships
between entities.