with

​

data as (

​

  select  null a, null b, null c from dual

  union all

​

  select  'a' a, null b, null c from dual

  union all

  select  null a, 'b' b, null c from dual

  union all

  select  null a, null b, 'c' c from dual

​

  union all

  select  'a' a, 'b' b, null c from dual

  union all

  select  'a' a, null b, 'c' c from dual

​

  union all

  select  null a, 'b' b, 'c' c from dual

​

  union all

  select  'a' a, 'b' b, 'c' c from dual

​

)

​

select a, b, c,trim(trailing ',' from case when a is null then '' else a||',' end ||case when b is null then '' else b||',' end || case when c is null then '' else c end)

col  

from data