DBMS GATEBOOK Exam_22

Question

In the following expressions of relational algebra, the relation R has schema R(a,b)

(A) Q1 and Q2 produce the same answer

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

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

(D) Q1 and Q2 produce different answers

Answer 1

Ans should be (C) ,take relation instance for this ques R(A,B) =(1,10),(2,10),(2,12),(3,4),(4,8),(4,3) and apply both query.

Answer 2

The relation has schema where the columns are 'a' and 'b'. From where did 'c' and 'd' come from ?

Comments

do u know renaming operation?

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@Pranabesh. Sorry. Just overlooked it. But the issue is what does  R.b$\neq$S.c mean? The remaming is done sequentially if anything not given. So, how can you compare 2 diff columns( if domain of 'a' and 'b' is diff) ?

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then it shold return empty set

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I doubt if its valid to compare 2 different domains. But if what you are saying is true then (A) is the answer.

COnsider relation:

a           b

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1         Arush

1         Megha

2        Arush

Both queries give 4 tuples:

1 Arush

1 Megha

2 Arush

2 Megha