SELECT
    items.id, items.name,
    COALESCE(
         (
            SELECT array_to_json(array_agg(row_to_json(subitems)))
            FROM (
                SELECT subitems.id, subitems.name
                FROM subitems
                WHERE subitems.item_id = items.id
            ) subitems
         ),
        '[]'
    ) AS subitems
FROM items