如何从wordpress中的wp_users表中获取所有user_nicename？

I have this function here:

function displaymeta(){
        global $post;
        global $wpdb;
        $authors = $wpdb->get_results("select id,user_nicename from $wpdb->users");
        foreach($authors as $auth){
                $authorname = get_the_author_meta('user_nicename',$auth->ID);
                echo $authorname;
        }
}
add_filter( 'the_content', 'displaymeta' );

It should retrieve all the user_nicename from wp_users table in database. But when I execute this code, I just get the first username twice, even though there are two distinct users. What am I missing here? Thanks!

UPDATE I need to do something like this:

 $arr=array();
 global $wpdb;
 $authors = $wpdb->get_results("SELECT ID, user_nicename from $wpdb->users 
                                ORDER BY display_name");
 foreach($authors as $auth)
 {
    $arr[]=get_author_meta('user_nicename', $auth->ID);
 }

I want to store the user_nicename in an array of all users in the wp_users table.

You can do it very simple like this:

$users = get_users();
foreach($users as $user) {
    echo $user->user_nicename;
}

More info here -> https://codex.wordpress.org/Function_Reference/get_users

If you simply want an array with just the user_nicenames you can do it like this:

global $wpdb

$user_nicenames = $wpdb->get_results("SELECT user_nicename FROM {$wpdb->prefix}users", ARRAY_A);

foreach($user_nicenames as $nice_name) {
    echo $nice_name;
}