Sql Query

Question

Let R(a,b) be a schema and Q1 and Q2 are queries on R.

Q1: SELECT * FROM R;

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

Which of the following statements is true?

(A) Q1 and Q2 produce the same answer.

(B) The answer to Q1 is always contained in the answer to Q2.

(C) The answer to Q2 is always contained in the answer to Q1.

(D) Q1 and Q2 produce different answers.

ans = C given

please explain the logic behind

Answer 1

Intersection is a set operation and set doesnt contain duplicate values. 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 Q2 duplicate rows are printed only once i.e there will be no repeatition. |Q2|<=|Q1| and hence result of Q2 will always be contained in Q1.

If instead of 'Intersect' we have used 'Intersect All' then duplicacy will be allowed and Q1 and Q2 will produce same result.

Answer 2

if it would have been INTERSECT ALL then answer would have been (A)

Answer 3

when your are using the intersection operator duplicates are removed so...

simply the first query gives u all the rows including duplicates

second query gives u all the rows from R without duplicates hence c is the answer