ABC Markets sell products to customers. The relational diagram shown in Figure P$10\mathrm{.}6$ represents the main entities for ABC's database. Note the following important characteristics:

A customer may make many purchases, each one represented by an invoice.

The CUS$\_$BALANCE is updated with each credit purchase or payment and represents the amount the customer owes.

The CUS$\_$BALANCE is increased $(+)$ with every credit purchase and decreased $()$ with every customer payment.

The date of last purchase is updated with each new purchase made by the customer.

The date of last payment is updated with each new payment made by the customer.

An invoice represents a product purchase by a customer.

An INVOICE can have many invoice LINEs, one for each product purchased.

The INV$\_$TOTAL represents the total cost of the invoice, including taxes.

The INV$\_$TERMS can be $"30,""60,"$ or $"90"($representing the number of days of credit$)$ or "CASH," "CHECK," or $"$CC$."$

The invoice status can be "OPEN," "PAID," or "CANCEL."

A product's quantity on hand $($P$\_$QTYOH$)$ is updated $($decreased$)$ with each product sale.