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 query/queries compute the names of sailors who have reserved a red boat?
Q1: π_sname((σ_color='red' Boats) ⋈ Reserves ⋈ Sailors)
Q2: π_sname(π_sid((π_bid σ_color='red' Boats) ⋈ Reserves) ⋈ Sailors)
Q3: π_sname((σ_color='red' Reserves) ⋈ Boats ⋈ Sailors)

Pick an option to check yourself, or open the answer below.

Show answer & explanation

Correct answer: A. Both Q1 and Q2

Explanation

Q1 joins red boats to their reservations and sailors. Q2 does the same with early projections. Q3 selects on color in Reserves, which has no color attribute.

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Which query/queries compute the names of sailors who have reserved a red boat?Q1: πsname((σcolor='red' Boats) ⋈ Reserves ⋈ Sailors)Q2: πsname(πsid((πbid σcolor='red' Boats) ⋈ Reserves) ⋈ Sailors)Q3: πsname((σcolor='red' Reserves) ⋈ Boats ⋈ Sailors)

Explanation

More Database Management Systems questions

Which query/queries compute the names of sailors who have reserved a red boat?
Q1: π_sname((σ_color='red' Boats) ⋈ Reserves ⋈ Sailors)
Q2: π_sname(π_sid((π_bid σ_color='red' Boats) ⋈ Reserves) ⋈ Sailors)
Q3: π_sname((σ_color='red' Reserves) ⋈ Boats ⋈ Sailors)