Oracle8i Java Stored Procedures Developer's Guide Release 2 (8.1.6) Part Number A81358-01 |
Library | Service | Contents | Index

## Drawing the Entity-Relationship Diagram

The objective is to develop a simple system for managing customer purchase orders. First, you must identify the business entities involved and their relationships. To do that, you draw an entity-relationship (E-R) diagram by following the rules and examples given in Figure 5-1.

#### Figure 5-1 Rules for Drawing an E-R Diagram

As Figure 5-2 illustrates, the basic entities in this example are customers, purchase orders, line items, and stock items.

#### Figure 5-2 E-R Diagram for Purchase Order Application

A `Customer` has a one-to-many relationship with a `Purchase` `Order` because a customer can place many orders, but a given purchase order can be placed by only one customer. The relationship is optional because zero customers might place a given order (it might be placed by someone not previously defined as a customer).

A `Purchase` `Order` has a many-to-many relationship with a `Stock` `Item` because a purchase order can refer to many stock items, and a stock item can be referred to by many purchase orders. However, you do not know which purchase orders refer to which stock items.

Therefore, you introduce the notion of a `Line` `Item`. A `Purchase` `Order` has a one-to-many relationship with a `Line` `Item` because a purchase order can list many line items, but a given line item can be listed by only one purchase order.

A `LineItem` has a many-to-one relationship with a `StockItem` because a line item can refer to only one stock item, but a given stock item can be referred to by many line items. The relationship is optional because zero line items might refer to a given stock item.

| Copyright © 1996-2000, Oracle Corporation. All Rights Reserved. |
Library | Service | Contents | Index