chapter2.4习题2.4.1

Exercise2.4.1(a-g) 欢迎指正

• What PC model have a speed of at least 3.00?
  πmodel(σspeed≥3.00(PC))
• Which manufactures make laptops with a hard disk of at least 100GB?
  P(maker,model)=σtype=′laptop′(Product)
  L(model,hd)=σhd≥100(Laptop)
  S(maker)=πmaker(P⋈L)
• Find the model number and price of all products(of any type) made by manufacturer B
  R1(model)=πmodel(σmaker=′B′(Product))
  R2(model,price)=πmodel,price(PC)
  R3(model,price)=πmodel,price(Laptop)
  R4(model,price)=πmodel,price(Printer)
  R5(model,price)=R2∪R3∪R4
  R6(maker)=πmaker(R1⋈R5)
• Find the model numbers of all color laser printers
  σcolor=′true′ANDtype=′laser′(Printer)
• Find those manufacturers that sell Laptops, but not PC
  R1(maker)=πmaker(σtype=′pc′(Product))
  R2(maker)=πmaker(σtype=′laptop′(Product))
  R3(maker)=R2−R1
• Find those hard-disk sizes that occur in two or more PC
  R1=ρR11(PC)×ρR12(PC)
  R2(hd)=πhd(σR11.hd=R12.hd∩R11.model≠R12.model(R1))
• Find those pairs of PC models that have both the same speed and RAM. A pair should be listed only once;e.g., list(i, j) but not (j, i)
  R1=ρPC1(PC)×ρPC2(PC)
  πPC1.model,PC2.model(σPC1.ram=PC2.ram∩PC1.speed=PC2.speed∩PC1.model>PC2.model(R1))