Untitled

Oracle: Concat with delimiter, but only if both operands are NOT NULL
select
  a, b, c,
  substr(abc, 1, length(abc) - 1) as abc
from
  (select
    a, b, c,
    nvl2(a, a || ';', '') || nvl2(b, b || ';', '') || nvl2(c, c || ';', '') as abc
  from
    Table1)

-- Some sample data, roughly equivalent to yours
with t as (
  select 'foo' as x from dual union all
  select null       from dual union all
  select 'bar'      from dual
)
-- Use the listagg aggregate function to join all values
select listagg(x, ';') within group (order by rownum)
from t;

-- I use SYS.ORA_MINING_VARCHAR2_NT as a TABLE TYPE. Use your own, if you prefer
select listagg(column_value, ';') within group (order by rownum)
from table(ORA_MINING_VARCHAR2_NT('foo', null, 'bar'));

select listagg(column_value, ';')
       within group (order by rownum)
from Table1
cross join table(ORA_MINING_VARCHAR2_NT(Table1.a, Table1.b, Table1.c))
group by Table1.id;

SELECT a
||     DECODE(b
       ,      NULL, NULL
       ,      ';' || b)
||     DECODE(c
       ,      NULL, NULL
       ,      ';' || c)
||     DECODE(d
       ,      NULL, NULL
       ,      ';' || d)
...
FROM   table1