Aggregate functions — Aggregations: GROUP BY and HAVING

From many rows to a single value

Until now every query returned one row per record. Aggregate functions do the opposite: they take a set of rows and summarize it into a single value.

Function	What it computes
`COUNT(*)`	number of rows
`COUNT(col)`	number of non-null values of `col`
`SUM(col)`	sum of the values
`AVG(col)`	average (mean) of the values
`MIN(col)`	minimum value
`MAX(col)`	maximum value

For example, over the products table:

SELECT COUNT(*) FROM products;        -- how many products are there?
SELECT AVG(price) FROM products;      -- average price of all of them
SELECT MIN(price), MAX(price) FROM products;

Without GROUP BY, the aggregate function acts on all the rows in the table (or those that survive the WHERE) and produces a single result row.

Aliases with AS

An aggregated column would literally be called AVG(price). To give it a readable name you use AS:

SELECT AVG(price) AS average_price, COUNT(*) AS total
FROM products;

COUNT(*) counts rows (including those with nulls); COUNT(col) counts only the rows where col is not NULL. That difference matters when there is incomplete data.