SELECT t.column_name
FROM user_tab_columns t
WHERE t.nullable = 'Y'
AND t.table_name = 'YOUR_TABLE_NAME'
AND t.num_distinct = 0
SELECT t.column_name
FROM user_tab_columns t
WHERE t.nullable = 'Y'
AND t.table_name = 'YOUR_TABLE_NAME'
AND t.num_distinct = 0