sql - 将空值视为唯一值

Question

我需要在列上计算不同的值，例如:

Hours
1
1
2
null
null
null

结果必须是: 3 。我的查询是:

select count(distinct hour) from hours;

但它返回:2.我还测试了:

select count(*) from hours group by hour

但它返回三行:

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

我如何将空值算作1值，并使用distinct避免计算重复值？

我正在学习高级SQL，他们希望我对所有解决方案有以下要求:

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

Answer 1

select  count(distinct col1) + count(distinct case when col1 is null then 1 end)
from    YourTable