SELECT orders.id AS id, string_agg(ordered_items.item_id :: CHARACTER VARYING, ', ') AS items FROM orders INNER JOIN ordered_items ON orders.id = ordered_items.order_id GROUP BY orders.id;
Standard input is empty
SELECT orders.id AS id, string_agg(ordered_items.item_id :: CHARACTER VARYING, ', ') AS items FROM orders INNER JOIN ordered_items ON orders.id = ordered_items.order_id GROUP BY orders.id;