GATE | GATE CS 2018 | Question 59

Consider the relations r(A, B) and s(B, C), where s.B is a primary key and r.B is a foreign key referencing s.B. Consider the query

Q: r⋈(σ_B<5(s))

Let LOJ denote the natural left outer-join operation. Assume that r and s contain no null values.

Which one of the following is NOT equivalent to Q?

(A) σ_B<5(r ⋈ s)
(B) σ_B<5(r LOJ s)
(C) r LOJ (σ_B<5(s))
(D) σ_B<5(r)LOJ s


Answer: (C)

Explanation: Since, we are joining/LOJ using attribute B which is primary key of table s and foreign key of table r.

So, we need to apply condition σ_B<5 on left table of join always, i.e., table r because left outer join (LOJ) returns all the values from an inner join plus all values in the left table that do not match to the right table.

So, option (C) is correct.

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