Question

If I use array_agg to collect names, I get my names separated by commas, but in case there is a null value, that null is also taken as a name in the aggregate. For example :

SELECT g.id,
       array_agg(CASE WHEN g.canonical = 'Y' THEN g.users ELSE NULL END) canonical_users,
       array_agg(CASE WHEN g.canonical = 'N' THEN g.users ELSE NULL END) non_canonical_users
FROM groups g
GROUP BY g.id;

it returns ,Larry,Phil instead of just Larry,Phil (in my 9.1.2, it shows NULL,Larry,Phil).

Instead, if I use string_agg(), it shows me only the names (without empty commas or nulls).

The problem is that I have Postgres 8.4 installed on the server, and string_agg() doesn’t work there. Is there any way to make array_agg work similar to string_agg() ?

Answer 1

select
    id,
    (select array_agg(a) from unnest(canonical_users) a where a is not null) canonical_users,
    (select array_agg(a) from unnest(non_canonical_users) a where a is not null) non_canonical_users
from (
    SELECT g.id,
           array_agg(CASE WHEN g.canonical = 'Y' THEN g.users ELSE NULL END) canonical_users,
           array_agg(CASE WHEN g.canonical = 'N' THEN g.users ELSE NULL END) non_canonical_users
    FROM groups g
    GROUP BY g.id
) s

Or, simpler and may be cheaper, using array_to_string which eliminates nulls:

SELECT
    g.id,
    array_to_string(
        array_agg(CASE WHEN g.canonical = 'Y' THEN g.users ELSE NULL END)
        , ','
    ) canonical_users,
    array_to_string(
        array_agg(CASE WHEN g.canonical = 'N' THEN g.users ELSE NULL END)
        , ','
    ) non_canonical_users
FROM groups g
GROUP BY g.id