GATE CS 2021 Set 1 — Question 37
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Databases → Relational Model → Joins
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Question
The following relation records the age of 500 employees of a company, where (indicating the employee number) is the key:Consider the following relational algebra expression:What does the above expression generate?
Correct answer
(C) Employee numbers of all employees whose age is not the minimum.
Solution
Let be the relation .
The expression is , where is a copy of renamed with attributes .The join condition selects tuples from where the employee's age is strictly greater than the age of some employee in (which is just ).
The expression is , where is a copy of renamed with attributes .The join condition selects tuples from where the employee's age is strictly greater than the age of some employee in (which is just ).
- If an employee has the minimum age in the company, there is no employee such that . Thus, employees with the minimum age will NOT be selected.
- If an employee has an age greater than the minimum, there exists at least one employee (the one with minimum age) such that . Thus, all such employees will be selected.
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