php - 表中的总数

我有下表，我需要得到在他们的考试中获得A的学生人数。这是我想从下表中实现的目标：
3A=0学生。2个A=3个学生。

+--------------+------------+------+
|  student_ID  | kod_subjek | gred |
+--------------+------------+------+
| 746123096687 | 02         | A    |
| 746123096687 | 12         | B    |
| 746123096687 | 21         | A    |
| 860206145454 | 12         | A    |
| 860206145454 | 02         | A    |
| 881012085535 | 02         | A    |
| 881012085535 | 21         | A    |
+--------------+------------+------+

我尝试：

mysql> SELECT student_ID, COUNT(gred) FROM data_exam GROUP BY student_ID;

输出为：

+--------------+-------------+
| student_ID   | COUNT(gred) |
+--------------+-------------+
| 746123096687 |           3 |
| 860206145454 |           2 |
| 881012085535 |           2 |
+--------------+-------------+

它不起作用。它只计算特定学生的所有分数。请帮我解决这个问题。谢谢您。

最佳答案

SELECT a_count, COUNT(*) AS cnt
FROM
(
  SELECT COUNT(*) AS a_count
  FROM data_exam
  WHERE gred = 'A'
  GROUP BY student_id
) x
GROUP BY a_count
ORDER BY a_count

Example on SQL-Fiddle返回：

a_count   cnt
      2     3