SUM adds the values of a column. AVG gives their average (the sum divided by the number of values).

SELECT SUM(stock), AVG(price) FROM products;

 sum |         avg
-----+---------------------
 810 | 21.1616666666666667

One SELECT can hold several aggregate functions. The result is still one row.

Round the average

An average often has many decimal places. Use ROUND (Chapter 3) around it:

SELECT ROUND(AVG(price), 2) AS avg_price FROM products;

An expression inside

The value inside the parentheses can be an expression. It runs for each row first:

SELECT SUM(quantity * unit_price) AS revenue FROM order_items;

NULL is skipped

Like COUNT(col), SUM and AVG skip NULL. They do not count it as 0.

To count a NULL as 0, replace it first: AVG(COALESCE(stars, 0)) returns (5 + 0 + 3) / 3, about 2.67.