In this exercise we introduce one of our running examples of $|$ a relational database schema. The database schema consists of four relations, whose schemas are: Product $($maker$,$ model, type$)$ PC$($model$,$ speed, ram, hd$,$ price$)$ Laptop$($model$,$ speed, ram, hd$,$ screen, price$)$ Printer$($model$,$ color, type, price$)$ The Product relation gives the manufacturer, model number and type $($PC$,$ laptop, or printer$)$ of various products. We assume for convenience that model numbers are unique over all manufacturers and product typ