AVG function in MySQL - Mini Project: Build & Apply

Calculate Average Product Price Using AVG Function

📖 Scenario: You work in a small store that sells different products. You want to find out the average price of all products to understand the general pricing.

🎯 Goal: Build a simple database table of products with their prices and write a query to calculate the average price using the AVG function.

📋 What You'll Learn

Create a table called products with columns id, name, and price

Insert exactly three products with given prices

Write a query to calculate the average price of all products using the AVG function

Use an alias average_price for the average value in the query result

💡 Why This Matters

🌍 Real World

Stores and businesses often need to analyze average prices to set competitive pricing or understand sales trends.

💼 Career

Knowing how to use aggregate functions like AVG is essential for data analysts, database administrators, and anyone working with data to summarize information quickly.

1

Create the products table

Write a SQL statement to create a table called products with three columns: id as an integer primary key, name as a variable character string of length 50, and price as a decimal number with two decimal places.

MySQL

-- Create the products table with id, name, and price columns
-- Your code here

Hint

Use CREATE TABLE with column definitions. Remember to set id as the primary key.

2

Insert product data into products

Insert three products into the products table with these exact values: (1, 'Pen', 1.20), (2, 'Notebook', 2.50), and (3, 'Eraser', 0.80).

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10, 2)
);
-- Insert the three products below
-- Your code here

Hint

Use a single INSERT INTO statement with multiple rows.

3

Write a query to calculate the average price

Write a SQL query that selects the average of the price column from the products table using the AVG function. Use the alias average_price for the result.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10, 2)
);

INSERT INTO products (id, name, price) VALUES
(1, 'Pen', 1.20),
(2, 'Notebook', 2.50),
(3, 'Eraser', 0.80);

-- Write the query to calculate average price below
-- Your code here

Hint

Use SELECT AVG(price) AS average_price FROM products; to get the average price.

4

Complete the project with a comment explaining the result

Add a SQL comment below the query explaining that the result shows the average price of all products in the table.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10, 2)
);

INSERT INTO products (id, name, price) VALUES
(1, 'Pen', 1.20),
(2, 'Notebook', 2.50),
(3, 'Eraser', 0.80);

SELECT AVG(price) AS average_price FROM products;
-- Your comment here

Hint

Add a comment starting with -- explaining the query result.

Practice

1. What does the AVG() function do in MySQL?

easy

A. Returns the sum of all values in a column

B. Counts the number of rows in a table

C. Finds the maximum value in a column

D. Calculates the average value of a numeric column

Solution

1. Step 1: Understand the purpose of AVG()

   The AVG() function is designed to calculate the average (mean) of numeric values in a column.

2. Step 2: Compare with other aggregate functions

   Unlike COUNT(), MAX(), or SUM(), AVG() specifically returns the average value.

3. Final Answer:

   Calculates the average value of a numeric column -> Option D

4. Quick Check:

   AVG() = average [OK]

Hint: AVG() always returns the mean of numeric values [OK]

Common Mistakes:

• Confusing AVG() with COUNT()
• Thinking AVG() sums values instead of averaging
• Assuming AVG() includes NULL values in calculation

2. Which of the following is the correct syntax to find the average salary from a table named employees?

easy

A. SELECT AVG(salary) FROM employees;

B. SELECT AVG salary FROM employees;

C. SELECT AVERAGE(salary) FROM employees;

D. SELECT salary AVG() FROM employees;

Solution

1. Step 1: Recall correct AVG() syntax

   The correct syntax uses AVG() as a function with the column name inside parentheses.

2. Step 2: Check each option

   SELECT AVG(salary) FROM employees; uses AVG(salary) correctly. Options A, C, and D have syntax errors or wrong function names.

3. Final Answer:

   SELECT AVG(salary) FROM employees; -> Option A

4. Quick Check:

   AVG(column) syntax [OK]

Hint: AVG() requires parentheses around the column name [OK]

Common Mistakes:

• Omitting parentheses in AVG()
• Using wrong function name like AVERAGE()
• Placing AVG() after column name incorrectly

3. Given the table scores with values:
id | score
1 | 80
2 | 90
3 | NULL
4 | 70
What will the query SELECT AVG(score) FROM scores; return?

medium

A. 80

B. NULL

C. 80.0

D. 75

Solution

1. Step 1: Identify values considered by AVG()

   AVG() ignores NULL values, so it averages 80, 90, and 70.

2. Step 2: Calculate average of non-NULL scores

   (80 + 90 + 70) / 3 = 240 / 3 = 80.0. MySQL's AVG() returns a floating-point value.

3. Final Answer:

   80.0 -> Option C

4. Quick Check:

   AVG ignores NULL, average = 80.0 [OK]

Hint: AVG ignores NULLs and returns float average [OK]

Common Mistakes:

• Including NULL in average calculation
• Expecting integer instead of float result
• Confusing AVG with SUM or COUNT

4. Consider this query:
SELECT AVG(price) FROM products WHERE price > 0;
It returns NULL even though there are products with positive prices. What is the likely cause?

medium

A. The price column contains only NULL values

B. The WHERE clause filters out all rows

C. AVG() cannot be used with WHERE clause

D. The table products does not exist

Solution

1. Step 1: Understand AVG() behavior with WHERE clause

   The WHERE clause filters rows before AVG() calculates the average.

2. Step 2: Analyze why AVG() returns NULL

   If the WHERE clause excludes all rows (no price > 0), AVG() has no values to average and returns NULL.

3. Final Answer:

   The WHERE clause filters out all rows -> Option B

4. Quick Check:

   Empty filtered rows cause AVG() to return NULL [OK]

Hint: Check WHERE filters; no rows means AVG() returns NULL [OK]

Common Mistakes:

• Assuming AVG() ignores WHERE clause
• Thinking AVG() returns 0 if no rows
• Believing AVG() cannot be combined with WHERE

5. You have a sales table with columns region and amount. How do you write a query to find the average sales amount per region, excluding regions with no sales?

hard

A. SELECT region, AVG(amount) FROM sales GROUP BY region HAVING AVG(amount) > 0;

B. SELECT region, AVG(amount) FROM sales WHERE amount IS NOT NULL GROUP BY region;

C. SELECT region, AVG(amount) FROM sales GROUP BY region WHERE amount > 0;

D. SELECT region, AVG(amount) FROM sales GROUP BY region HAVING amount > 0;

Solution

1. Step 1: Group sales by region

   Use GROUP BY region to calculate averages per region.

2. Step 2: Filter out regions with no sales

   Use HAVING AVG(amount) > 0 to exclude groups where average is NULL or 0 (no sales).

3. Step 3: Check other options

   SELECT region, AVG(amount) FROM sales WHERE amount IS NOT NULL GROUP BY region; (SELECT region, AVG(amount) FROM sales WHERE amount IS NOT NULL GROUP BY region;) filters NULL rows before grouping but includes regions where non-NULL amounts average to 0. Options C and D have syntax errors or misuse WHERE with GROUP BY.

4. Final Answer:

   SELECT region, AVG(amount) FROM sales GROUP BY region HAVING AVG(amount) > 0; -> Option A

5. Quick Check:

   Use HAVING to filter groups by AVG() [OK]

Hint: Use HAVING to filter groups after GROUP BY [OK]

Common Mistakes:

• Using WHERE instead of HAVING to filter groups
• Not excluding NULL averages
• Misplacing WHERE clause after GROUP BY