使用count（*）函数

Question

有人可以使用count（*）query ??

编写类似的查询

因为此查询不采用空值

select count(b.Difficulty_level) 
From cr_answer_table AS b, cr_response_table AS c
where b.Difficulty_level like 'Medium%' AND b.Q_No = c.Q_No And b.Correct_answer = c.Answer_Marked
group by CR_Id