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Unit 2: Relational Query Languages

20212m

Which of the following operations is used if we are interested in only certain columns of a table?

  • (i) PROJECTION
    (ii) SELECTION
    (ii) UNION
    (iv) JOIN
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Answer: (i) PROJECTION

Explanation: Projection (pi) operates on columns - selects specific attributes vertically, eliminating duplicates. Selection (sigma) operates on rows - selects tuples satisfying condition. Union and Join combine relations. Example: pi_{name,age}(Student) returns only name and age columns.

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