SQL query - single text value from group by

Hi

I am trying to work out if this is possible in a select query

I have a group by query which could result in several rows, what I want
to do is do a text equivalent of a sum() eg:

SELECT sum(inv_id),date,cust
from invoice
group by date,cust

Is there any way to get to a single concatenated inv_id field with just
one row?

thanks