How does the GROUP BY operator handle NULL values?

The GROUP BY operator treats NULL values as a separate group. All rows with NULL in the grouping column are included in this single NULL group.

Example:

Suppose we have a products table with columns category (String) and price (double):

category	price
Electronics	100
Electronics	200
NULL	50
Clothing	30
NULL	70

When executing the query:

SELECT category, AVG(price)
FROM products
GROUP BY category;

The result will be approximately:

category	AVG(price)
Electronics	150.0
NULL	60.0
Clothing	30.0