SQL Aggregate Functions Explained — COUNT, SUM, AVG, MIN, MAX

Quick answer

SQL aggregate functions summarize data across multiple rows and return a single value.

The most common ones are:

COUNT() → how many rows
SUM() → total value
AVG() → average value
MIN() → smallest value
MAX() → largest value

Tip: If your question sounds like “how many,” “total,” or “average,” you probably need an aggregate function.

In this guide

Sample table COUNT() SUM() AVG() MIN() and MAX() Using with GROUP BY How NULL values behave When to use them Common mistakes

Sample table

Imagine this employees table:

employee_id	name	department	salary	city
1	Ana	Sales	62000	Chicago
2	Ben	IT	78000	Dallas
3	Chris	HR	56000	Chicago
4	Dana	IT	85000	Denver
5	Evan	Sales	59000	Austin

We’ll use this table for all examples so you can clearly see how each function works.

COUNT()

COUNT() tells you how many rows exist.

SELECT COUNT(*) AS total_employees
FROM employees;

This returns the total number of rows in the table.

COUNT(*) counts all rows, even if some columns contain NULL values.

SUM()

SUM() adds up numeric values.

SELECT SUM(salary) AS total_salary
FROM employees;

This returns the total salary across all employees.

Use SUM() for totals like revenue, expenses, quantities, or salaries.

AVG()

AVG() calculates the average value.

SELECT AVG(salary) AS avg_salary
FROM employees;

This returns the average salary.

AVG() ignores NULL values when calculating the average.

MIN() and MAX()

MIN() returns the smallest value. MAX() returns the largest value.

SELECT MIN(salary) AS lowest_salary,
       MAX(salary) AS highest_salary
FROM employees;

This shows the lowest and highest salaries in the table.

Using aggregate functions with GROUP BY

Aggregate functions become much more powerful when combined with GROUP BY.

SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;

This returns one row per department with a count of employees.

Think of it as: “Group first, then summarize each group.”

How NULL values behave

COUNT(*) counts all rows
COUNT(column) ignores NULL values
SUM() and AVG() ignore NULL values

NULL does not mean zero. It means “no value,” and most aggregate functions skip it.

When to use aggregate functions

Use COUNT: number of users, orders, or rows

Use SUM: total revenue, total cost, total quantity

Use AVG: average salary, average order value

Use MIN/MAX: lowest/highest values

If your question involves summarizing data instead of listing individual rows, use aggregate functions.

Common mistakes

Using aggregate functions without understanding GROUP BY
Mixing normal columns and aggregates without grouping properly
Forgetting that NULL values are ignored
Using COUNT(column) and expecting it to count NULL rows

If you combine normal columns with aggregate functions, SQL usually requires those columns to be included in GROUP BY.

Practice questions

1. Count how many employees exist

2. Find total salary across all employees

3. Find average salary

4. Find highest and lowest salary

5. Count employees per department using GROUP BY

Bottom line

Aggregate functions let you summarize data quickly and answer high-level questions.

Quick memory trick: COUNT = how many, SUM = total, AVG = average, MIN/MAX = extremes.

Quick answer

In this guide

Sample table

COUNT()

SUM()

AVG()

MIN() and MAX()

Using aggregate functions with GROUP BY

How NULL values behave

When to use aggregate functions

Common mistakes

Practice questions

Bottom line

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