Missing field through a derived table in SQL

Your subquery doesn't include post_id in its selection. Include that field in the subquery this way:

SELECT wc_order_items.order_id AS Order_Number
FROM wc_order_items
LEFT JOIN (
    SELECT meta_value AS Given_Name, post_id -- include this
    FROM wc_metadata
    WHERE meta_key = '_shipping_first_name'
) AS shipping
ON wc_order_items.order_id = shipping.post_id
WHERE wc_order_items.order_id = 3276

I see the solution does work and was accepted, but the query can be written much more neatly. Tried with mysql and postgres.

SELECT orders.order_id AS Order_Number
FROM wc_order_items AS orders
LEFT JOIN wc_metadata AS metadata
  ON orders.order_id = metadata.post_id
 AND metadata.meta_key = '_shipping_first_name'
WHERE orders.order_id = 3276