How to left join intermediate result table after filter ta(subquery)?
I'm not sure I understand your problem, but judging from your sample data you can use a conditional count:

SELECT COUNT(CASE WHEN stu_name like 'A%' then 1 end) AS A
     , COUNT(CASE WHEN stu_name like 'B%' then 1 end) AS B
FROM stuInfo
WHERE stu_no = 1;


CASE WHEN stu_name like 'A%' then 1 end evaluates to 1 if stu_name starts with A and null otherwise. COUNT counts all non-null expressions.