GO Classes Test Series 2024 | Mock GATE | Test 12 | Question: 25

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GO Classes Test Series 2024 | Mock GATE | Test 12 | Question: 25

GO Classes asked Jan 21 • retagged Jan 25 by Lakshman Bhaiya

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Let $\mathrm{R}(\mathrm{a}, \mathrm{b})$ be a schema and $\mathrm{Q} 1$ and $\mathrm{Q} 2$ are queries on $\mathrm{R}$.

Q1: SELECT * FROM R;
Q2: (SELECT * FROM R) INTERSECT (SELECT * FROM R);

Which of the following statements is true?

$\text{Q1}$ and $\text{Q2}$ produce the same answer.
The answer to $\text{Q1}$ is always contained in the answer to $\text{Q2}.$
The answer to $\text{Q2}$ is always contained in the answer to $\text{Q1}.$
$\text{Q1}$ and $\text{Q2}$ produce different answers.

GO Classes asked Jan 21 • retagged Jan 25 by Lakshman Bhaiya

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I;ve made the mistake by just forgetting the basic concept of set operation which makes removal of duplicate rows.i.e. intersection is a set operation .so duplicate rows are removed .And that’s the reason why Q1 covers all the rows of Q2. — Srken, Jan 23
@Srken Does R(a,b) itself indicates that it is in 1NF right? So there will not be duplicates tuples in R(a,b)?? — Chandu559, Jan 26
yeah but SQL tables can have duplicate tuples and they can be printed — Srken, Jan 26
$ \large{\colorbox{yellow}{Detailed video solution of this question with direct time stamp}}$
All India Mock Test 3 - Solutions Part 2 — GO Classes Support, Jan 27

GO Classes · Answer 1 · 2024-01-21T01:20:55+0000

The intersection is a set operation and set doesnt contain duplicate values. $\text{Q1}$ will print all the rows including those which are duplicates i.e there may be some rows in its output that might be repeated. In $\text{Q2}$ duplicate rows are printed only once i.e there will be no repetition. $\text{Q2} \leq \text{Q1}$ and hence result of $\text{Q2}$ will always be contained in $\text{Q1}$.

If it would have been INTERSECT ALL then the answer would have been (A).

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GO Classes Test Series 2024 | Mock GATE | Test 12 | Question: 25

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