This is the previous question to help answer question 4 below:

This is from my textbook Information Modeling and Relational Databases. This is out of section 6.3 Value Constraints and Independent Types.

4. Consider the previous question, but suppose that the same item may appear on more than one line of the same invoice. Devise an alternative identification scheme to deal with this situation. (Hint: Compare this with the compact disc example. Sometimes you need to identify things in terms of their position.)

An Australian software retailer, SoftMart, maintains an information system to help with invoice and stock control. It has recently opened for business, and has made only a few sales So far, The details of the softwarc tems it has in stock are shown. The softwarc items are standardly identified by item codes, but also have unique titles. There are exactly three software categories, identified by codes (SS spreadsheet, DB database, WP word. processor. The full names of these c egories are not recorded. The list price of an ilem is the normal price at which the item is currently sold. However, SofuMart may sell an ilem at less than the current list price (e.g., SoftMart may give a discount for bulk orders or to fa vored clients, and the list price itself may change with time). There is no rule to enable the unit price (i.c., the actual price charged for a copy of an item) to be derived from the current list price Category Stock Listprice Item code Title 799.50 B123 Blossom 123 SS DataLight 10 DL DB 700.00 DB3 1999.99 Database 3 DB 400.00 Quinquo SS SQL SQL plus 1890.50 DB TS TextStar WP 500.00 WL Word Light WP 10 700.00 Customers are identified by a customer number, but the combination of their name and address is also unique. For simplicity, customer name is treated as a single character string, as is address. Customers have at most one phone number recorded. The next table shows customer details. Address Phone Customer# Name 3765000 001 Starcorp 5 Sun St. St Lucia 4067 002 Eastpac 30 Beach Rd, Sandgate 4017 269 1111 003 Dr I.N. Stein 7 Sesame St, St Lucia 4067 An Australian software retailer, SoftMart, maintains an information system to help with invoice and stock control. It has recently opened for business, and has made only a few sales So far, The details of the softwarc tems it has in stock are shown. The softwarc items are standardly identified by item codes, but also have unique titles. There are exactly three software categories, identified by codes (SS spreadsheet, DB database, WP word. processor. The full names of these c egories are not recorded. The list price of an ilem is the normal price at which the item is currently sold. However, SofuMart may sell an ilem at less than the current list price (e.g., SoftMart may give a discount for bulk orders or to fa vored clients, and the list price itself may change with time). There is no rule to enable the unit price (i.c., the actual price charged for a copy of an item) to be derived from the current list price Category Stock Listprice Item code Title 799.50 B123 Blossom 123 SS DataLight 10 DL DB 700.00 DB3 1999.99 Database 3 DB 400.00 Quinquo SS SQL SQL plus 1890.50 DB TS TextStar WP 500.00 WL Word Light WP 10 700.00 Customers are identified by a customer number, but the combination of their name and address is also unique. For simplicity, customer name is treated as a single character string, as is address. Customers have at most one phone number recorded. The next table shows customer details. Address Phone Customer# Name 3765000 001 Starcorp 5 Sun St. St Lucia 4067 002 Eastpac 30 Beach Rd, Sandgate 4017 269 1111 003 Dr I.N. Stein 7 Sesame St, St Lucia 4067