Output this by authors?

So there is this code example in the codex,

<?php
// set the meta_key to the appropriate custom field meta key
$meta_key = 'miles';
$allmiles = $wpdb->get_var( $wpdb->prepare(
	"
		SELECT sum(meta_value)
		FROM $wpdb->postmeta
		WHERE meta_key = %s
	",
	$meta_key
) );
echo "<p>Total miles is {$allmiles}</p>";
?>

But what if you are trying to find the total miles for an individual author? Where the custom field is contained in every post they’ve ever made?