SELECT item, 
            decode(NO, 1, grade, '소계'),
            sum(m_qty), sum(m_amt),
            sum(s_qty),  sum(s_amt)
FROM ( SELECT item, grade,
                       sum(m_qty)   m_qty, 
                       sum(m_amt)  m_amt,
                       sum(s_qty)    s_qty,  
                       sum(s_amt)   s_amt
             F ROM TAB1
             WHERE yymm = :in_daye
                 AND saup = :saup
       GROUP BY item, grade
      ) x,
          COPY_T y
 WHERE y.NO <= 2
GROUP BY item, NO,
           decode(NO, 1, grade, '소계') -- 소계결과값 bottom
           decode(NO, 1, '소계', grade) -- 소계결과값 top
;

decode(NO, 1, grad, '소계') 로 할경우...

decode(NO, 1, '소계', grade) 로 할경우...

왜 차이점이 생기는건가요?