Selection operation implementation 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 filters rows from a table based on a specified condition.

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

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

D It creates a new table by adding columns from two tables.

💡 Hint

Think about how you pick certain rows from a table based on a rule.

The selection operation chooses rows from a table that satisfy a given condition, effectively filtering the data.

📋 Factual

intermediate

SQL Equivalent of Selection Operation

Which SQL clause is used to perform the selection operation on a table?

A GROUP BY

B ORDER BY

C WHERE

D HAVING

💡 Hint

This clause filters rows before grouping or ordering.

The WHERE clause filters rows in SQL, similar to the selection operation in relational algebra.

🚀 Application

advanced

Result of Selection Operation with Multiple Conditions

Given a table Employees with columns id, name, and salary, what is the result of the selection operation with condition salary > 50000 AND name LIKE 'J%'?

DBMS Theory

SELECT * FROM Employees WHERE salary > 50000 AND name LIKE 'J%';

A All employees with salary above 50000 regardless of name.

B All employees with salary above 50000 or names starting with 'J'.

C All employees with salary below 50000 and names starting with 'J'.

D All employees with salary above 50000 whose names start with 'J'.

💡 Hint

AND means both conditions must be true.

The selection filters rows where both salary is greater than 50000 and the name starts with 'J'.

🔍 Analysis

advanced

Effect of Selection Operation on Table Size

If a selection operation is applied on a table with 1000 rows using a condition that matches 10% of the rows, how many rows will the resulting table have?

A 100 rows

B 10 rows

C 900 rows

D 1000 rows

💡 Hint

Calculate 10% of 1000.

The selection filters rows, so only 10% of 1000 rows remain, which is 100 rows.

❓ Reasoning

expert

Identifying the Correct Selection Query Output

Consider a table Products with columns product_id, category, and price. Which query will return all products in the 'Electronics' category priced below 300?

A SELECT * FROM Products WHERE category = 'Electronics' OR price < 300;

B SELECT * FROM Products WHERE category = 'Electronics' AND price < 300;

C SELECT * FROM Products WHERE category = 'Electronics' AND price > 300;

D SELECT * FROM Products WHERE category != 'Electronics' AND price < 300;

💡 Hint

Both conditions must be true for selection.

Only option B correctly filters products that are in 'Electronics' and priced below 300.

Practice

1. What is the main purpose of the SELECT statement with a WHERE clause in a database?

easy

A. To change the structure of a table

B. To delete rows from a table

C. To add new columns to a table

D. To retrieve only rows that meet specific conditions

Solution

1. Step 1: Understand the role of SELECT

   The SELECT statement is used to get data from a table.

2. Step 2: Understand the role of WHERE clause

   The WHERE clause filters rows to include only those that meet given conditions.

3. Final Answer:

   To retrieve only rows that meet specific conditions -> Option D

4. Quick Check:

   SELECT + WHERE = filtered rows [OK]

Hint: WHERE filters rows; SELECT retrieves data [OK]

Common Mistakes:

• Confusing WHERE with DELETE
• Thinking WHERE adds columns
• Believing WHERE changes table structure

2. Which of the following is the correct syntax to select all columns from a table named Employees where the Age is greater than 30?

easy

A. SELECT * FROM Employees WHERE Age > 30;

B. SELECT * Employees WHERE Age > 30;

C. SELECT FROM Employees WHERE Age > 30;

D. SELECT * FROM Employees AGE > 30;

Solution

1. Step 1: Check SELECT syntax

   The correct syntax starts with SELECT, then columns or *, then FROM table name.

2. Step 2: Check WHERE clause syntax

   WHERE must be followed by a condition like Age > 30.

3. Final Answer:

   SELECT * FROM Employees WHERE Age > 30; -> Option A

4. Quick Check:

   Correct SELECT + FROM + WHERE syntax [OK]

Hint: SELECT * FROM table WHERE condition; [OK]

Common Mistakes:

• Omitting FROM keyword
• Placing WHERE before FROM
• Missing semicolon at end

3. Consider the table Products with columns ProductID, Name, and Price. What will be the result of this query?

SELECT Name FROM Products WHERE Price <= 50;

medium

A. All product names regardless of price

B. All product names with price less than or equal to 50

C. All product names with price greater than 50

D. An error because Price <= 50 is invalid

Solution

1. Step 1: Understand the SELECT clause

   The query selects only the Name column from the Products table.

2. Step 2: Understand the WHERE condition

   The condition Price <= 50 filters rows to those with price 50 or less.

3. Final Answer:

   All product names with price less than or equal to 50 -> Option B

4. Quick Check:

   WHERE Price <= 50 filters products [OK]

Hint: WHERE filters rows by condition; SELECT picks columns [OK]

Common Mistakes:

• Confusing <= with >= operator
• Expecting all products without filter
• Thinking query causes error

4. Identify the error in the following SQL query:

SELECT * FROM Customers WHERE City = 'New York'

medium

A. Missing FROM keyword

B. Incorrect use of single quotes around string

C. Missing semicolon at the end

D. WHERE clause should be after ORDER BY

Solution

1. Step 1: Check SQL syntax completeness

   SQL statements should end with a semicolon to mark the end.

2. Step 2: Verify other parts

   FROM keyword is present, single quotes around string are correct, WHERE comes before ORDER BY.

3. Final Answer:

   Missing semicolon at the end -> Option C

4. Quick Check:

   SQL statements end with ; [OK]

Hint: Always end SQL statements with a semicolon [OK]

Common Mistakes:

• Forgetting semicolon
• Misplacing WHERE clause
• Using double quotes instead of single quotes

5. You have a table Orders with columns OrderID, CustomerID, and Status. You want to select all orders that are either 'Pending' or 'Processing'. Which SQL query correctly implements this selection?

hard

A. SELECT * FROM Orders WHERE Status IN ('Pending', 'Processing');

B. SELECT * FROM Orders WHERE Status = 'Pending' AND 'Processing';

C. SELECT * FROM Orders WHERE Status = 'Pending' OR 'Processing';

D. SELECT * FROM Orders WHERE Status = 'Pending', 'Processing';

Solution

1. Step 1: Understand the condition for multiple values

   To select rows where Status matches multiple values, use IN or multiple OR conditions.

2. Step 2: Compare options

   IN ('Pending', 'Processing') is correct and concise. OR requires full conditions like Status = 'Pending' OR Status = 'Processing'. A lone string after OR like 'Processing' makes the condition always true, selecting extra rows. AND between values or commas cause syntax errors.

3. Final Answer:

   SELECT * FROM Orders WHERE Status IN ('Pending', 'Processing'); -> Option A

4. Quick Check:

   Use IN for multiple values in WHERE [OK]

Hint: Use IN for multiple OR conditions in WHERE [OK]

Common Mistakes:

• Using AND instead of OR
• Incorrect syntax with commas in WHERE
• Not using quotes around string values