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.

Practice

1. What does a row represent in a database table?

easy

A. A group of tables linked together

B. The name of the table

C. The type of data stored in the table

D. A single record containing data for each column

Solution

1. Step 1: Understand the structure of a table

   A table is made up of rows and columns where columns define data types and rows hold data.

2. Step 2: Identify what a row represents

   A row is one complete record with values for each column in the table.

3. Final Answer:

   A single record containing data for each column -> Option D

4. Quick Check:

   Row = single record [OK]

Hint: Rows = records, columns = data types [OK]

Common Mistakes:

• Confusing rows with columns
• Thinking rows are table names
• Mixing up rows with database schemas

2. Which of the following is the correct way to select all columns from a table named Employees?

easy

A. GET * FROM Employees;

B. SELECT ALL COLUMNS Employees;

C. SELECT * FROM Employees;

D. SELECT COLUMNS FROM Employees;

Solution

1. Step 1: Recall SQL syntax for selecting all columns

   The standard SQL command to select all columns is SELECT * FROM table_name;.

2. Step 2: Match the correct syntax

   SELECT * FROM Employees; matches the correct syntax exactly.

3. Final Answer:

   SELECT * FROM Employees; -> Option C

4. Quick Check:

   SELECT * = all columns [OK]

Hint: Use * to select all columns [OK]

Common Mistakes:

• Using GET instead of SELECT
• Writing SELECT ALL COLUMNS which is invalid
• Omitting FROM keyword

3. Given the table Students with columns id, name, and age, what will this query return?

SELECT name FROM Students WHERE age > 20;

medium

A. Only the names of students older than 20

B. All columns for students older than 20

C. Only the ages of students older than 20

D. An error because age is not selected

Solution

1. Step 1: Analyze the SELECT clause

   The query selects only the name column from the table.

2. Step 2: Analyze the WHERE condition

   It filters rows where age is greater than 20, so only those students are included.

3. Final Answer:

   Only the names of students older than 20 -> Option A

4. Quick Check:

   SELECT name WHERE age > 20 = names only [OK]

Hint: SELECT columns control output, WHERE filters rows [OK]

Common Mistakes:

• Thinking all columns are returned
• Assuming age column is returned without selecting it
• Believing query causes error due to WHERE clause

4. What is wrong with this SQL query?

SELECT id, name FROM Students WHERE age = 'twenty';

medium

A. Using quotes around a number in WHERE clause causes error

B. Comparing age to a string 'twenty' may cause no matches

C. Selecting multiple columns requires commas

D. Missing FROM keyword before table name

Solution

1. Step 1: Check the WHERE clause comparison

   The query compares age to the string 'twenty', but age is numeric.

2. Step 2: Understand effect of comparing number to string

   This will not cause a syntax error but likely returns zero rows because no numeric age equals 'twenty'.

3. Final Answer:

   Comparing age to a string 'twenty' may cause no matches -> Option B

4. Quick Check:

   Wrong data type comparison = no results [OK]

Hint: Match data types in WHERE clause [OK]

Common Mistakes:

• Thinking quotes cause syntax error
• Assuming query returns matching rows
• Ignoring data type mismatch

5. You have a table Orders with columns order_id, customer_id, and amount. You want to find the total amount spent by each customer. Which SQL query correctly groups and sums the data?

hard

A. SELECT customer_id, SUM(amount) FROM Orders GROUP BY customer_id;

B. SELECT order_id, SUM(amount) FROM Orders GROUP BY order_id;

C. SELECT customer_id, amount FROM Orders WHERE SUM(amount) > 100;

D. SELECT customer_id, amount FROM Orders GROUP BY amount;

Solution

1. Step 1: Identify the goal

   We want total amount spent per customer, so we must group by customer_id.

2. Step 2: Use aggregation function

   SUM(amount) calculates total amount per group.

3. Step 3: Check query correctness

   SELECT customer_id, SUM(amount) FROM Orders GROUP BY customer_id; correctly groups by customer_id and sums amount.

4. Final Answer:

   SELECT customer_id, SUM(amount) FROM Orders GROUP BY customer_id; -> Option A

5. Quick Check:

   Group by customer_id + SUM(amount) = total per customer [OK]

Hint: Group by key, aggregate values with SUM [OK]

Common Mistakes:

• Grouping by wrong column
• Using WHERE with aggregate functions incorrectly
• Selecting columns not in GROUP BY without aggregation