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The figure shows the entity relationships among the following four relational entities or tables:

• Customer
• Purchase Order
• Line Item of a purchase order
• Stock Item

A customer places zero or more purchase orders, but a purchase order has at most one customer.

A purchase order can contain many line items, but a given line item occurs on exactly one purchase order.

Each line item refers to exactly one stock item; a stock item can occur in zero or many line items.