Postgres: Concat multiple columns, while including nulls - postgresql

Question

I have a table with four columns that I need to concat. Two of the columns contain some NULL values.
I need the result to contain separators indicating all four columns, like so:
colA,colB,colC,colD
or if one column (here colB) was null,
colA,,colC,colD
I can't seem to find a clean way to do this. The best I have come up with is:
concat_ws(colA, COALESCE(colB, ''), COALESCE(colC, ''), colD, ',')
This feels cumbersome (especially because I need to do this repeatedly). Is there a better way?

Answer 1

Since the final value can't be NULL, you don't need to worry about preserving them. Just use an empty string. This is how you'd write it in standard SQL.
select coalesce(cola, '') || ', ' ||
coalesce(colb, '') || ', ' ||
coalesce(colc, '') || ', ' ||
coalesce(cold, '')
from your_table_name;
The concat_ws() function doesn't skip empty strings, but it does skip null columns. That means you still have to use coalesce().
select concat_ws(', ', coalesce(cola, ''),
coalesce(colb, ''),
coalesce(colc, ''),
coalesce(colb, ''))
from your_table_name;

Answer 2

I think you should use a cursor and then a loop also, in order to show null values you must use a nvl() function, like this:
PROCEDURE TODO IS
vAux in VARCHAR2;
CURSOR CUR_TEST IN
SELECT NVL(colb, 'replace_with'), colA, colD
FROM DUAL;
Begin
for x in CUR_TEST loop
vAux := x.colb||','||x.colA||','||x.colD;
end loop;
end TODO;
For checking what nvl() does: http://www.techonthenet.com/oracle/functions/nvl.php