retrieve wordpress post-meta using php not wordpress functions

The reverse mechanism should work. I assume you are trying the get post metadata with the post row.

$resultat = mysqli_query($con,"SELECT * FROM wp_postmeta 
                                 LEFT JOIN wp_post 
                                 ON wp_postmeta.post_id = wp_post.ID 
                                 WHERE wp_post.post_type="shop_order"");

However, there will be a lots of duplication. If one post have 100 metadata, you are also getting the post row 100 times.