The following relation records the age of 500 employees of a company, where empNo (indicating the employee number) is the key: empAge(empNo,\ age) Consider the following relational algebra expression: _empNo≤ft(empAge _(age > age1) _empNo1,\, age1(empAge)) What does the above expression generate?

Employee numbers of all employees whose age is not the minimum.

Employee numbers of only those employees whose age is the maximum.

Employee numbers of only those employees whose age is more than the age of exactly one other employee.

Employee numbers of all employees whose age is not the minimum.

Employee numbers of all employees whose age is the minimum.

GATE CSE 2021 Set 1 Q37: Meaning of a self theta-join on employee age

The following relation records the age of 500 employees of a company, where $empNo$ (indicating the employee number) is the key:

$empAge(\underline{empNo},\ age)$

Consider the following relational algebra expression:

$\Pi_{empNo}\left(empAge \bowtie_{(age > age1)} \rho_{empNo1,\, age1}(empAge)\right)$

What does the above expression generate?

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Answer: C (official GATE 2021 final answer key)

Concept used: A self theta-join with $age > age1$ keeps an employee whenever at least one other employee is younger.

First step: Ask which employees have no partner row with a smaller age.

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Source: official GATE 2021 Set 1 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key