Unit 4 · Database Management Systems Paper 2 CS · September 2022

Directions (Q31–35): Consider the relational schema of Sailors S, Reserves R and Boats B.

Table 1: Sailors

Sid	Sname	Rating	Age
22	Dustin	7	45.0
29	Brutus	1	33.0
31	Lubber	8	55.5
32	Andy	8	25.5
58	Rusty	10	35.5
64	Horatio	7	35.5
71	Zorba	10	16.0
74	Horatio	9	35.0
85	Art	3	25.5
95	Bob	3	63.5

Table 2: Reserves

Sid	Bid	Day
22	101	10/10/98
22	102	10/10/98
22	103	10/8/98
22	104	10/7/98
31	102	11/10/98
31	103	11/6/98
31	104	11/12/98
64	101	9/5/98
64	102	9/8/98
74	103	9/8/98

Table 3: Boats

Bid	Bname	Color
101	Interlake	blue
102	Interlake	red
103	Clipper	green
104	Marine	red

Which relational algebra query computes the sids of sailors with age over 20 who have not reserved a red boat?

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Correct answer: A. π_sid(σ_age>20 Sailors) − π_sid((σ_color='red' Boats) ⋈ Reserves ⋈ Sailors)

Explanation

Take all sailors older than 20 and subtract those who reserved some red boat. B selects an impossible condition, C uses age < 20, and ∧ is not a relational operator.

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Which relational algebra query computes the sids of sailors with age over 20 who have not reserved a red boat?

Explanation

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