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GATE CSE 2024 Session 2 Q45: Cross products needed for cities with at least 3 residents

Question

The relation schema, Person(pid, city), describes the city of residence for every person uniquely identified by pid. The following relational algebra operators are available: selection, projection, cross product, and rename.

To find the list of cities where at least 3 persons reside, using the above operators, the minimum number of cross product operations that must be used is

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Answer: B (official GATE 2024 final answer key)

Concept used: Without aggregation, "at least kk" needs kk copies of the relation compared pairwise.

First step: Take three renamed copies of Person and select rows with equal city and pairwise distinct pid; count the products needed to combine three copies.

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Source: official GATE 2024 Session 2 CS question paper and final answer key, organised by IISc Bangalore. Question paper · Answer key

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