Wordpress: Returns an array of all meta values over all posts of a certain type based on a key, can be unique or not

get_meta_values.php

<?php 

/**
 * Returns an array of all meta values over all posts of a certain type based on a key, can be unique or not
 */
function nigelheap_get_meta_values($key = '', $type = 'post', $status = 'publish', $unique = true) {

    global $wpdb;

    if( empty( $key ) )
        return;

    $distinct = '';
    if($unique){
    	$distinct = 'DISTINCT';
    }

    $r = $wpdb->get_col( $wpdb->prepare( "
        SELECT {$distinct} pm.meta_value FROM {$wpdb->postmeta} pm
        LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
        WHERE pm.meta_key = '%s' 
        AND p.post_status = '%s' 
        AND p.post_type = '%s' 
    ", $key, $status, $type ) );

    return $r;
}