Solution

The correct answer is: 4) Only Q2

Explanation:

Q1: πsname ((δbid = 103 Boats) ⋈ Sailors)

This query first selects the rows from the Boats table where Bid is 103 (δ_bid = ₁₀₃ Boats). Then, it performs a natural join (⋈) with the Sailors table. Finally, it projects (π) the attribute Sname. However, this query is incorrect because it selects the sailors based on the boat's Bid instead of the Reserve table.

Q2: πsname (δbid = 103 (Reserves ⋈ Sailors))

This query first performs a natural join (⋈) between the Reserves and Sailors tables. Then, it selects the rows where Bid is 103 (δ_bid = 103). Finally, it projects (π) the attribute Sname. This query correctly retrieves the names of sailors who have reserved boat 103.

Q3: πsname ((δbid = 103 Reserves) ⋈ Sailors)

This query first selects the rows from the Reserves table where Bid is 103 (δ_bid = ₁₀₃ Reserves). Then, it performs a natural join (⋈) with the Sailors table. Finally, it projects (π) the attribute Sname. Although this query performs the join correctly, it mistakenly selects rows based on the Bid in the Reserves table instead of the Bid in the join result.

Therefore, the only correct query that computes the name of sailors who have reserved boat 103 is Q2.