Include NULL values in aggregate function COUNT() in MySQL

count.mysql

CREATE TABLE example_table (
    hour INT
);

INSERT INTO example_table (hour) 
VALUES (1),(2),(2),(NULL),(NULL),(NULL);

COMMIT;

SELECT 
    COUNT(DISTINCT hour) + (CASE
        WHEN COUNT(hour) <> COUNT(*) THEN 1
        ELSE 0
    END) AS 'distinct hours'
FROM
    example_table;
SELECT 
    hour, COUNT(*) AS 'count'
FROM
    example_table
GROUP BY hour;

SQL Fiddle: http://sqlfiddle.com/#!9/2b326d/1