Oracle: Concat with delimiter, but only if both operands are NOT NULL

select trim(';' from REGEXP_REPLACE (a || ';' || b || ';' || c , ';+' , ';')) abc 
from Table1

AFAIK, there's no succinct way to do this.

In the past, I've resorted to

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

but that's no better than your example.

I know you're using 10g, so that won't work. But for completeness, LISTAGG() handles NULL values "correctly". For that you'd have to update to 11g2, though:

-- 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;

Or a bit more succinct, if you want to list columns from a table:

-- 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'));

Or against an actual table:

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;

Now I'm not sure if this is so much better (more readable) than your original example :-)