UGC NET CSE | November 2017 | Part 3 | Question: 11

Question

Given two relations $R_1(A,B)$ and $R_2(C,D)$, the result of following query

Select distinct A, B  
From R1, R2

Is guaranteed to be same as R₁ provided one of the following condition is satisfied.

• A. $R_1$ has no duplicates and $R_2$ is empty.
• B. $R_1$ has no duplicates and $R_2$ is non-empty.
• C. Both $R_1$ and $R_2$ have no duplicates.
• D. $R_2$ has no duplicates and $R_1$ is non – empty.

Comments

question framed is wrong.

in 2nd option it should be R1 has no duplicates and R2 is non-empty.

so. option 2 is correct i think

---

Why R2 shoud be nonempty what if it is empty?

---

https://gateoverflow.in/672/gate2000-2-25

Answer 1

1. R1 has no duplicates and R2 is empty. ( If R2 is empty and we select distinct A,B from R1,R2 ,then cartesian product would result in zero tuples)
2. R2 has no duplicates and R1 is empty (If R1 is empty, then by no means the query will getch any content from R1)
3. Both R1 and R2 have no duplicates. (This will fetch distinct columns from R2 containing null values)
4. R2 has no duplicates and R1 is non empty. ( Correct Answer : The query selects all attributes of R1 Since we have distinct in query, result can be equal to R1 only if R1 doesn’t have duplicates.)

Comments

@Sayan Could you please explain the third and fourth point with the help of examples.

---

In point 4, suppose R1 has duplicates then also we will get the result as equal to R1?

In my opinion R1 should have no duplicate.

Another doubt, If we are taking cartesian product of R1 and R2 as in case above and R2 is empty then what will be the result.

Please reply.

---

Why not option A is the answer?Please explain.

Answer 2

Select A,B
From R₁, R₂ :
In this query first we will take cartesian product of R₁, R₂ (R₂ must be non empty)for this 0- R₂ then select distinct A,B from Cartesian product of R₁, R₂ (for A,B being distinct there should not any duplicate A,B).
And
Select A,B
From R₂
For this query A,B → No duplicate A,B.
By combining condition for both query we will get the right condition.
So, option (B) is correct.