GATE CSE 2001 | Question: 1.25

Question

Let r and s be two relations over the relation schemes R and S respectively, and let A be an attribute in R.  The relational algebra expression $\sigma_{A=a}(r \bowtie s)$ is always equal to

• A. $\sigma_{A=a}(r)$
• B. $r$
• C. $\sigma_{A=a}(r) \bowtie s$
• D. None of the above

Comments

what is ‘a’ in above question?

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@UltraRadiantX 

 ‘a’ is some value in the ‘A’ attribute.

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thanks to the early use of selection

Answer 1

Answer is C.

C is just the better form of query, more execution friendly because requires less memory while joining. query, given in question takes more time and memory while joining.

Comments

@Abhrajyoti00 is this complete pdf is for gate relevance ?

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@shikhar500 No, just read about the equivalence rules

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Thanks to the early enrollment in GO classes !!

Answer 2

 option c

Comments

what is "relation schemes R and S respectively" in this question??

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why you have taken B attribute in relation r and how come B & C attribute

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What if B dont have entry

20,Y?

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@srestha mam 

R and S are → Relation schema defines the design and structure of the relationship like it consist of the relation name, set of attributes/field names/column names. every attribute would have an associated domain. (In simple you can think of this as a relations name).

r and s are relation instances. The set of tuples of a relation at a particular instance of time is called a relation instance.

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Abhay Agrawal

yes, the last table last row will be → a 20 y.

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your sigma looks like 6

Answer 3

Answer is (C).

It is an efficient way to write the query to select first the tuples then cross with other relation.