I have two string columns
select a,b from foo
select a || b from foo
select a||', '||b from foo
No operator matches the given name and argument type(s). You might
need to add explicit type casts.
The problem was in nulls in the values; then the concatenation does not work with nulls. The solution is as follows:
SELECT coalesce(a, '') || coalesce(b, '') FROM foo;