Tables, rows, and columns concept in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Identify the number of rows returned

Given the table Employees with columns id, name, and department, what is the number of rows returned by this query?

SELECT * FROM Employees WHERE department = 'Sales';

SQL

SELECT * FROM Employees WHERE department = 'Sales';

A 0

B 5

C 3

D 7

💡 Hint

Think about how many employees belong to the Sales department.

The query filters rows where the department is 'Sales'. There are exactly 3 such rows in the Employees table.

🧠 Conceptual

intermediate

Understanding columns in a table

Which of the following best describes a column in a database table?

A A single record containing all data about one entity

B A temporary storage area for query results

C A horizontal set of data values representing multiple entities

D A vertical set of data values representing a specific attribute

💡 Hint

Think about how data is organized vertically in tables.

A column holds data for one attribute across all rows, arranged vertically.

📝 Syntax

advanced

Identify the correct SQL syntax to select specific columns

Which SQL query correctly selects only the name and department columns from the Employees table?

A SELECT name, department FROM Employees;

B SELECT * FROM Employees WHERE name, department;

C SELECT name & department FROM Employees;

D SELECT name department FROM Employees;

💡 Hint

Remember the syntax for selecting multiple columns uses commas.

Option A uses correct SQL syntax with commas separating column names.

🔧 Debug

advanced

Find the error in this SQL query

What error will this SQL query produce?

SELECT id, name, department FROM Employees WHERE department = Sales;

A Syntax error due to missing quotes around 'Sales'

B No error, query runs successfully

C Runtime error due to missing table Employees

D Syntax error due to missing comma between columns

💡 Hint

String values in SQL must be enclosed in quotes.

The value Sales is a string and must be enclosed in single quotes to avoid syntax error.

❓ optimization

expert

Optimize query to count rows efficiently

Which query is the most efficient way to count the number of employees in the Employees table?

A SELECT COUNT(id, name) FROM Employees;

B SELECT COUNT(*) FROM Employees;

C SELECT * FROM Employees; -- then count rows in application

D SELECT COUNT(department) FROM Employees WHERE department IS NOT NULL;

💡 Hint

Use SQL aggregate functions designed for counting rows.

COUNT(*) counts all rows efficiently without filtering or extra conditions.