volume list query

$meta_query = array(
	'relation' => 'AND',
	array(
		'key'     => 'parent_book',
		'value'   => 'book_id',
		'compare' => 'LIKE',
		'type'    => 'CHAR',
	),
	array(
		'relation' => 'OR',
		array(
			'key'     => 'volume_number',
			'compare' => 'EXISTS',
		),
		array(
			'key'     => 'volume_number',
			'compare' => 'NOT EXISTS',
		),
	),
);