Counting null values as unique value

Question

I need to count different values on a column, such as:

Hours 1 1 2 null null null 

The result must be: 3. My query is:

select count(distinct hour) from hours; 

but it returns: 2. I tested also:

select count(*) from hours group by hour 

but it returns three rows:

(1) 3 (2) 2 (3) 1 

How can I count null values as 1 value and use distinct to avoid count repeated values?

I'm learning advanced SQL, they want me these requirements for all the solutions:

Try to minimize the number of subqueries you need to solve the query. Furthermore, you are not allowed to use the following constructions:

• SELECT in the FROM or SELECT. You are allowed to have subqueries (SELECT in the WHERE or HAVING)
• Combinations of aggregation functions such as COUNT (COUNT. ..)), SUM (COUNT. ..)) and the like.
• UNION if you can avoid it.
• Non-standard functions (such as NVL)
• CASE