SQL LEFT JOIN Subquery Alias

I'm running this SQL query:

SELECT wp_woocommerce_order_items.order_id As No_Commande
FROM  wp_woocommerce_order_items
LEFT JOIN 
	(
		SELECT meta_value As Prenom
		FROM wp_postmeta
		WHERE meta_key = '_shipping_first_name'
	) AS a
ON wp_woocommerce_order_items.order_id = a.post_id
WHERE  wp_woocommerce_order_items.order_id =2198

And I get this error:

> #1054 - Unknown column 'a.post_id' in 'on clause'.

I think my code is pretty simple, but I can't make it right. What am I doing wrong?

You didn't select post_id in the subquery. You have to select it in the subquery like this:

SELECT wp_woocommerce_order_items.order_id As No_Commande
FROM  wp_woocommerce_order_items
LEFT JOIN 
    (
        SELECT meta_value As Prenom, post_id  -- <----- this
        FROM wp_postmeta
        WHERE meta_key = '_shipping_first_name'
    ) AS a
ON wp_woocommerce_order_items.order_id = a.post_id
WHERE  wp_woocommerce_order_items.order_id =2198 

I recognize that the answer works and has been accepted but there is a much cleaner way to write that query. Tested on mysql and postgres.

SELECT wpoi.order_id As No_Commande
FROM  wp_woocommerce_order_items AS wpoi
LEFT JOIN wp_postmeta AS wpp ON wpoi.order_id = wpp.post_id 
                            AND wpp.meta_key = '_shipping_first_name'
WHERE  wpoi.order_id =2198