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Midterm Solution of Fall 2009 Fall 2009 Solution only

Computer Science
Course Code
Ihab Ilyas
Study Guide

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Question 2 Part A
Relational Algebra: Take the cross product of the pickup table with itself
and ensure two different reservations exist from the same customer. Solu-
tions that assumed that a dropoff entry was required were given full credit
as well.
ρ(P1, P ickup)
ρ(P2,(cnum :cnum2, rnum :rnum2), P ickup)
ρ(T woremtals, Πcnum(σcnum=cnum2rnum6=rnum2(P1×P2)))
Answer: Πcnum,cname(C ustomer ./ T woremtals)
SQL: Solutions can use the technique similar to the relational alegbra ex-
pression or use the aggregate functions provided by SQL:
SELECT cnum, cname
FROM Customer INNER JOIN Pickup ON Pickup.cnum = Customer.cnum
GROUP BY cnum, cname
Question 2 Part B
Relational Algebra: Currently renting implies that there should be an entry
for the customer in the pickup table but not in the dropoff table:
ρ(CurrentRentals, Πrnum(P ickup)Dropof f )
ρ(W aterlooC ustomers, Πcnum,cname(σcity=W aterloo(C ustomer))
ρ(T oyotas2009,Πlicense(σmake=T oyota year=2009 (Car))
Answer: Πcnum,cname(P ickup ./ C urrentRentals ./ W aterlooC ustomers ./
T oyotas2009)
SQL: Use NOT EXISTS / EXCEPT or similar clauses to get eliminate pick-
ups that do not exist in the dropoff table:
SELECT cnum, cname
FROM Customer
INNER JOIN Pickup ON Pickup.cnum = Customer.cnum
INNER JOIN Car ON Pickup.license = Car.license
WHERE make = ’Toyota’
AND year = 2009
AND city = ’Waterloo’
AND Pickup.rnum NOT IN (SELECT rnum from Dropoff )
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