How to order by post meta name in wp admin?

I am trying to order by post meta name which I added for post. Post meta is “_merchant_id” an I know how to sort by merchan_id but I don’t know how to sort by merchant name.

I am sorting by merchant id like this:

public function column_orderby( $vars ) {
        if ( isset( $vars['orderby'] ) && is_admin() ) {
            switch ( $vars['orderby'] ) {
case 'merchant':
 $vars = array_merge( $vars, array(
     'meta_key' => '_merchant_id',
     'orderby' => 'meta_value_num'
 ) );

break;
...

Merchant is another post type (“merchant”).
How can I sort by merchant name?