1. fieldcatalog의 집계계산을 할 field에

  slis_fieldcatalog_wa-do_sum = 'X'.

2. display Function 위에

  DATA : l_sort_it TYPE slis_t_sortinfo_alv, "부분합, 소트
         l_sort TYPE slis_sortinfo_alv.

  l_sort-spos = '1'.
  l_sort-fieldname = 'KUNNR'.
  l_sort-up = 'X'.
  l_sort-subtot = 'X'.

  APPEND l_sort TO l_sort_it.

3. display Function에

  it_sort                  = l_sort_it

하면 됩니당~

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