COUNT column Function

The COUNT column function returns the total number of non-NULL values in the column or expression, as the following example shows: 
SELECT COUNT (item_num) FROM items;

The ALL keyword can precede the specified column name for clarity, but the query result is the same whether you include the ALL keyword or omit it.

The following example shows how to include the ALL keyword in the COUNT column function: 
SELECT COUNT (ALL item_num) FROM items;