GATE CS 2015 Set 1 — Question 31
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Databases → Relational Model → DML Commands
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Question
SELECT operation in SQL is equivalent to
Correct answer
(D) the projection operation in relational algebra, except that SELECT in SQL retains duplicates
Solution
In Relational Algebra, the projection operation () selects specific columns from a relation and removes duplicate rows (since relations are sets). In SQL, the
SELECT clause is used to specify columns to be retrieved, which corresponds to projection. However, unlike relational algebra, the standard SQL SELECT statement retains duplicate rows in the result (multiset semantics) unless the DISTINCT keyword is used. Therefore, the SQL SELECT operation is equivalent to the projection operation in relational algebra, except that it retains duplicates.Continue learning with Success Tracker
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