WITH myTab AS
(
SELECT '919680010' item FROM dual
UNION ALL
SELECT '919680010-1' FROM dual
UNION ALL
SELECT '919680010-1/95' FROM dual
UNION ALL
SELECT '919680010-1/90' FROM dual
UNION ALL
SELECT '919680010-1/75' FROM dual
UNION ALL
SELECT '919680013-4/20' FROM dual
UNION ALL
SELECT '419560297/10' FROM dual
UNION ALL
SELECT '419560298/15' FROM dual
)
SELECT DISTINCT SUBSTR(item, 1, DECODE(INSTR(item, '/') -1, -1, LENGTH(item), INSTR(item, '/') -1)) FROM myTab;

SQL> WITH DATA AS(
  2  SELECT '919680010' ITEM_CODE FROM dual UNION ALL
  3  SELECT '919680010-1' FROM dual UNION ALL
  4  SELECT '919680010-1/95' FROM dual UNION ALL
  5  SELECT '919680010-1/90' FROM dual UNION ALL
  6  SELECT '919680010-1/75' FROM dual UNION ALL
  7  SELECT '919680013-4/20' FROM dual UNION ALL
  8  SELECT '419560297/10' FROM dual UNION ALL
  9  SELECT '419560298/15' FROM dual)
 10  SELECT CASE
 11            WHEN REGEXP_LIKE(ITEM_CODE, '-') AND REGEXP_LIKE(ITEM_CODE, '/') THEN
 12             SUBSTR(ITEM_CODE, 1, INSTR(ITEM_CODE, '/') - 1)
 13            WHEN REGEXP_LIKE(ITEM_CODE, '/') THEN
 14             SUBSTR(ITEM_CODE, 1, INSTR(ITEM_CODE, '/') - 1)
 15            ELSE
 16             ITEM_CODE
 17         END ITEM_CODE
 18    FROM DATA
 19   GROUP BY CASE
 20               WHEN REGEXP_LIKE(ITEM_CODE, '-') AND
 21                    REGEXP_LIKE(ITEM_CODE, '/') THEN
 22                SUBSTR(ITEM_CODE, 1, INSTR(ITEM_CODE, '/') - 1)
 23               WHEN REGEXP_LIKE(ITEM_CODE, '/') THEN
 24                SUBSTR(ITEM_CODE, 1, INSTR(ITEM_CODE, '/') - 1)
 25               ELSE
 26                ITEM_CODE
 27            END;
 
ITEM_CODE
--------------
919680010
419560297
919680010-1
919680013-4
419560298