AVG function in MySQL - Step-by-Step Execution

Concept Flow - AVG function

Start with a column of numbers

↓

Sum all values in the column

↓

Count how many values are in the column

↓

Divide sum by count

↓

Return the average value

The AVG function calculates the average by adding all values in a column and dividing by the number of values.

Execution Sample

MySQL

SELECT AVG(score) FROM tests;

This query calculates the average score from the 'tests' table.

Execution Table

Step	Action	Values Involved	Result
1	Identify column 'score' values	[80, 90, 70, 100, 60]	Values collected
2	Sum all values	80 + 90 + 70 + 100 + 60	400
3	Count number of values	5	5
4	Divide sum by count	400 / 5	80
5	Return average	80	80

💡 All values processed; average calculated as 80

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
sum	0	400	400	400	400
count	0	0	5	5	5
average	undefined	undefined	undefined	80	80

Key Moments - 2 Insights

Why does AVG ignore NULL values in the column?

AVG only sums and counts non-NULL values, so NULLs do not affect the sum or count, as shown in the execution_table where only actual numbers are processed.

What happens if the column has zero rows?

If there are no rows, the count is zero, so AVG returns NULL because dividing by zero is undefined. This is not shown in the current table but is important to remember.

Visual Quiz - 3 Questions

Look at the execution table, what is the sum of the 'score' values at Step 2?

A 5

B 80

C 400

D 4000

💡 Hint

Check the 'Result' column at Step 2 in the execution_table.

At which step does the AVG function calculate the final average value?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look for the division operation in the execution_table.

If one score was NULL, how would the count change in the variable_tracker?

A Count would decrease by 1

B Count would stay the same

C Sum would increase

D Average would be NULL

💡 Hint

Recall that AVG ignores NULLs, so count only includes non-NULL values.

Concept Snapshot

AVG(column_name)
Calculates the average of non-NULL values in a column.
Steps: sum values, count values, divide sum by count.
Returns NULL if no non-NULL values.
Useful for finding mean values in data.

Full Transcript

The AVG function in SQL calculates the average value of a numeric column. It works by first collecting all the non-NULL values in the column, then summing these values, counting how many there are, and finally dividing the sum by the count. For example, if the scores are 80, 90, 70, 100, and 60, the sum is 400 and the count is 5, so the average is 80. AVG ignores NULL values and returns NULL if there are no non-NULL values to average. This process helps find the mean value in a dataset easily.

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