作業3 參考解答

(a) A PC with a processor speed less than 2.00 must not sell for more than $500.

<latex> \sigma_{speed<2.00 \wedge price>500}(PC)= \emptyset </latex>

(b) A laptop with a screen size less than 15.4 inches must have at least a 100 gigabyte hard disk or sell for less than $1000.

<latex> \sigma_{screen<15.4 \wedge hd<100 \wedge price \geq 1000} (Laptop) = \emptyset </latex>

(c) No manufacturer of PC's may also make laptops.

<latex> \pi_{maker}(Product \bowtie PC) \bigcap \pi_{maker}(Product \bowtie Laptop) = \emptyset </latex>

(e) If a laptop has a larger main memory than a PC, then the laptop must also have a higher price than the PC.

<latex> \sigma_{Laptop.ram > PC.ram \wedge Laptop.price \leq PC.price}(Laptop \times PC) = \emptyset </latex>