📊 Aggregate Functions (COUNT, SUM, AVG, MIN, MAX)

Aggregate functions are used to perform calculations on multiple rows of a table and return a single value. They're mostly used with GROUP BY, but also work standalone.

1. COUNT()
Returns the number of rows.

Example:

SELECT COUNT(*) FROM employees;

Counts all employees in the table.

You can also count only non-null values in a column:

SELECT COUNT(email) FROM customers;


2. SUM()
Adds up all the values in a numeric column.

Example:

SELECT SUM(salary) FROM employees;

Gives you the total salary payout.


3. AVG()
Calculates the average value of a numeric column.

Example:

SELECT AVG(price) FROM products;

Finds the average product price.


4. MIN()
Returns the lowest value.

Example:

SELECT MIN(salary) FROM employees;

Finds the smallest salary.


5. MAX()
Returns the highest value.

Example:

SELECT MAX(salary) FROM employees;

Finds the highest salary in the table.


Bonus Example:

SELECT
COUNT(*) AS total_orders,
SUM(amount) AS total_revenue,
AVG(amount) AS avg_order_value
FROM orders;

This gives you a quick business summary: number of orders, total revenue, and average order value.


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