I would like to copy the meta_value from row to another (in the same table) where the user_id matches.

Each user in the `wp_usermeta` table has a `meta_key`/`meta_value` pair for `first_name` and a `meta_key`/`meta_value` pair for `shipping_first_name`. I would like to copy the `shipping_first_name` value to the `first_name` value where the `user_id` matches.

How should this MySQL query be written?