GATE CS 2024 Set 2 — Question 45
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Databases → Relational Model → Selection & Projection
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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 isCorrect answer
(B) 2
Solution
To find cities with at least 3 persons, we need to find three distinct persons such that they all live in the same city.
In relational algebra, this is achieved by joining the
Since we need to combine 3 tables, we require cross product operations.
In relational algebra, this is achieved by joining the
Person relation with itself. Since only cross product (Cartesian product) is available (along with selection and projection), we perform the join manually.We need 3 instances of the Person relation, say .1.Perform . This is the 1st cross product.
2.Perform . This is the 2nd cross product.
After obtaining , we apply selection to ensure:projection to get the city.Since we need to combine 3 tables, we require cross product operations.
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