Having trouble using the replace() function

Question

Hey everybody. :)
I've written a query which finds the address for a company (from 5 columns and concatenates them into one). These columns are street, town, county, telno and faxno.
Some of the companies have a null value for the county, so when I write the query it returns a result like... 3 Main Street, Manchester, , 0384758948, 04857846927, but if there was a county it would be 3 Main Street, Manchester, Greater Manchester, 0384758948, 04857846927.
I've been trying to write the query so if there isn't a county, it just removes the comma and the address becomes 3 Main Street, Manchester, 0384758948, 04857846927... but I just can't seem to figure it out so I thought I would come and ask the professionals. :)
set linesize 100
column suppname heading 'Supplier Name'
select suppname, street || ', ' || town || ', ' || county || ', ' || postcode || ', ' || telno || ', ' || faxno "Supplier Address"
from Suppliers
where county is null
replace(county, county, '');
This gives me a 'command not ended properly' error, but I can't for the life of me figure out why.
Thank you in advance. :)

Answer 1

with t as (
select '3 Main Street' street,
       'Manchester' town,
       null county,
       0384758948 postcode,
       0485784692 telno
  from dual
union
select '3 Main Street' street,
       'Manchester' town,
       'Greater Manchester' county,
       0384758948 postcode,
       0485784692 telno
  from dual)
select nvl2(street, street, null) ||
       nvl2(town, ', ' || town, null) ||
       nvl2(county, ', ' || county, null) ||
       nvl2(postcode, ', ' || postcode, null) ||
       nvl2(telno, ', ' || telno, null)
  from t