Selection operation in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Selection Operation in Relational Algebra

Which of the following best describes the selection operation in relational algebra?

A It extracts rows from a table that satisfy a given condition.

B It combines two tables by matching rows based on a common attribute.

C It removes duplicate rows from a table.

D It rearranges the columns of a table in a specified order.

💡 Hint

Think about filtering data based on conditions.

The selection operation filters rows that meet a specific condition, returning only those rows.

📋 Factual

intermediate

SQL Equivalent of Selection Operation

Which SQL clause corresponds to the selection operation in relational algebra?

A GROUP BY

B ORDER BY

C WHERE

D HAVING

💡 Hint

It filters rows before grouping or ordering.

The WHERE clause filters rows based on conditions, just like selection in relational algebra.

🚀 Application

advanced

Result of Selection Operation on a Table

Given a table Employees with columns EmpID, Name, and Salary, what will be the result of the selection operation σ_{Salary > 50000}(Employees)?

A Only the salary column for employees.

B All employees with salary less than or equal to 50000.

C All employees regardless of salary.

D All employees with salary greater than 50000.

💡 Hint

Selection filters rows based on the condition.

The selection operation returns rows where the salary is greater than 50000.

🔍 Analysis

advanced

Effect of Multiple Conditions in Selection

Consider the selection operation σ_{Age > 30 AND Department = 'Sales'}(Employees). What does this operation return?

A Employees older than 30 and in the Sales department.

B Employees older than 30 or in the Sales department.

C Employees younger than 30 and not in Sales.

D Employees in Sales regardless of age.

💡 Hint

AND means both conditions must be true.

The selection returns only employees who satisfy both conditions simultaneously.

❓ Reasoning

expert

Selection Operation and Result Cardinality

If a relation R has 1000 rows, and the selection condition is very restrictive so that only 10 rows satisfy it, what is the cardinality (number of rows) of the result of the selection operation σ_condition(R)?

A 1000 rows

B 10 rows

C 0 rows

D Cannot be determined without more information

💡 Hint

Selection filters rows that meet the condition.

The selection returns only the rows that satisfy the condition, so the result has 10 rows.