We’ve already covered how to use the GROUP BY clause and some aggregation functions like SUM(), AVG(), MAX(), MIN(), COUNT(). In this article, we will explain how the GROUP BY clause works when NULL values are involved. We will also explain about using NULLs with the ORDER BY clause. In SQL, NULL is a special marker used to indicate that a data value does not exist in the database. For more details, check out Wikipedia’s explanation of NULL in SQL.