AVG function in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the AVG function do in SQL?

The AVG function calculates the average (mean) value of a numeric column in a table.

beginner

How do you use the AVG function to find the average price from a products table?

Use: SELECT AVG(price) FROM products; This returns the average of all values in the price column.

beginner

Can the AVG function be used with non-numeric columns?

No, AVG only works with numeric columns like integers or decimals. Using it on text columns will cause an error.

intermediate

What happens if the AVG function is used on a column with NULL values?

NULL values are ignored by AVG. It calculates the average only from non-NULL values.

intermediate

How can you find the average salary per department using AVG?

Use GROUP BY: SELECT department, AVG(salary) FROM employees GROUP BY department; This shows average salary for each department.

What does the SQL AVG function return?

A The average value of a numeric column

B The total sum of a numeric column

C The highest value in a column

D The number of rows in a table

AVG calculates the mean (average) of numeric values in a column.

Which SQL query correctly finds the average age from a table named users?

A SELECT AVG(age) FROM users;

B SELECT SUM(age) FROM users;

C SELECT COUNT(age) FROM users;

D SELECT MAX(age) FROM users;

AVG(age) returns the average age; SUM, COUNT, and MAX do different calculations.

How does AVG handle NULL values in a column?

A It causes an error

B It treats NULL as zero

C It ignores NULL values

D It counts NULL as one

AVG skips NULLs and calculates average only from existing numeric values.

Can AVG be used on a text column?

A Yes, but only if text is numeric strings

B No, AVG only works on numeric columns

C Yes, AVG works on any column

D Only if text is converted first

AVG requires numeric data types; text columns cause errors.

Which SQL clause is used with AVG to get averages per group?

A HAVING

B ORDER BY

C WHERE

D GROUP BY

GROUP BY groups rows so AVG can calculate averages for each group.

Explain how the AVG function works and how it treats NULL values.

Think about how you find an average in real life and what happens if some data is missing.

Describe how to use AVG with GROUP BY to find averages per category.

Imagine you want average scores per class in a school.

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