GATE CS 2014 Set 3 — Question 40
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Databases → Relational Model → Joins
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Question
Consider the relational schema given below, where eId of the relation dependent is a foreign key referring to empId of the relation employee. Assume that every employee has at least one associated dependent in the dependent relation.employee (empId, empName, empAge)
dependent (depId, eId, depName, depAge)Consider the following relational algebra query:The above query evaluates to the set of empIds of employees whose age is greater than that of
dependent (depId, eId, depName, depAge)Consider the following relational algebra query:The above query evaluates to the set of empIds of employees whose age is greater than that of
Correct answer
(D) all of his/her dependents.
Solution
The query can be broken down as follows:
This implies that for all of their dependents, the condition must be false, i.e., .Thus, the query returns employees whose age is greater than that of all of his/her dependents.
1.: Returns all employee IDs.
2.: Returns the IDs of employees who have at least one dependent such that the employee's age is less than or equal to the dependent's age.
Subtracting set (2) from set (1) gives the IDs of employees who do NOT satisfy the condition in (2).So, we are looking for employees who do not have any dependent with .This implies that for all of their dependents, the condition must be false, i.e., .Thus, the query returns employees whose age is greater than that of all of his/her dependents.
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