第二章习题与答案(第三版).doc

Exercise 2.3.1 着个练习，今年新加的，我这边还没有做，你能帮忙做一下吗？ Exercise 2.4.1 a), f), h) Product (maker, model, type) PC(model, speed, ram, hd, price) Laptop(model,speed,ram,hd,screen,price) Printer(model,color,type,price) Write expressions of relational algebra to answer the following queries. For the data of Figs2.20 and 2.21, show the result of your query. a) what PC models have a speed of at least 3.00？ ( Model ( ((speed =3.00) (PC)) Result: 1005 1006 1013 f) Find those hard-disk sizes that occur in two or more PC’s. R1 = (pc1 (PC) R2 = (pc2 (PC) R3 = R1 (-JOIN (pc1.hd = pc2.hd and pc1.model pc2.model) R2 R4 = ( pc1.hd (R3) Result: 250 80 160 h) Find those manufacturers of at least two different computers (PC’s or laptops) with speeds of at least 2.8 R1 = (model,speed (PC) ( ( model,speed (Laptop) R2 = (Maker,model ( ((Speed = 2.8 (R1) natural JOIN Product)) R3 = ( T(maker2,model2)(R2) R4 = R2 theta JOIN maker=maker2 AND model model2 (R3) Answer(maker) = (Maker (R4) Result: B,E Exercise 2.5.1 b) Product (maker, model, type) PC(model, speed,ram,hd,rd,price) Laptop(model,speed,ram,hd,screen,price) Printer(model,color,type,price) Write your constraints either as containments or by equating an expression to the empty set. For the data of exercise 2.4.1, indicate any violations to your constraints. 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 (screen 15.4 AND hd 100 AND price = $1000 (Laptop) = ( Or: (screen 15.4(Laptop) ( (hd = 100 OR price 1000 (Laptop) Violation: Model speed ram hd screen price 2004 2.00 512 60 13.3 1150 No manufacturer of PC’s may also make laptops 需要验证 Pi make (PC natural join Product) intersection Pi maker (Laptop natural join Product) = ( A manufacturer of a PC must also make a laptop with at least as great a processor speed. (没有要求做。) This complex expression is best seen as a sequence of steps in which we define temporary relations R1 through R4 th