Joining arrays within group by clause

Question

We have a problem grouping arrays into a single array. We want to join the values from two columns into one single array and aggregate these arrays of multiple rows.

Answer 1

select n, array_agg(c) as c
from (
    select n, unnest(array[c1, c2]) as c
    from t
) s
group by n

Or simpler

select
    n,
    array_agg(c1) || array_agg(c2) as c
from t
group by n

---

To address the new ordering requirement:

select n, array_agg(c order by id, o) as c
from (
    select
        id, n,
        unnest(array[c1, c2]) as c,
        unnest(array[1, 2]) as o
    from t
) s
group by n