Any NULLS in the column on which the aggregate function is operating are ignored for the purposes of the function except COUNT(*), which includes them. If all the values in a column are NULL, COUNT(column_name) returns 0.

If no rows meet the conditions specified in the WHERE clause, COUNT returns a value of 0. The other functions all return NULL. Here are examples:

SELECT COUNT( DISTINCT Name )
   FROM Products
   WHERE UnitPrice > 50;

SELECT AVG( UnitPrice )
   FROM Products
   WHERE UnitPrice > 50;