SELECT NVL(등급, 'TOT_CNT') 등급
     , COUNT(DECODE(reg_day, '201103', 1)) "201103"
     , COUNT(DECODE(reg_day, '201102', 1)) "201102"
  FROM (SELECT reg_day
             , CASE WHEN study >= 65                THEN 'A'
                    WHEN study >= 50 AND study < 65 THEN 'B'
                    WHEN study >= 20 AND study < 50 THEN 'C'
                    WHEN                 study < 20 THEN 'D'
                END 등급
          FROM table a
         WHERE reg_day IN ('201103', '201102')
        )
 GROUP BY ROLLUP(등급)
;

마농님 정말 감사합니다.

계속 안되서 끙끙되고 있었는데 금새 해결해주셨네요 ㅠㅠ

정말감사합니다.