Java Stored Procedures Developer's Guide
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.
As Figure 5-2 illustrates, the basic entities in this example are customers, purchase orders, line items, and stock items.
Customer has a one-to-many relationship with a
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).
Order has a many-to-many relationship with a
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
Order has a one-to-many relationship with a
Item because a purchase order can list many line items, but a given line item can be listed by only one purchase order.
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.