return all WordPress user meta_key through the database

get all wordpress user meta_key.php

<?php

/**
* Returns all unique meta key from user meta database
*
* @param no parameter right now 
* 2 pieces of attractive WordPress code
*/

/**
* The first code example:
*/

function get_user_meta_key() {
   global $wpdb;

    $select = "SELECT distinct $wpdb->usermeta.meta_key FROM $wpdb->usermeta";
    
    $usermeta = $wpdb->get_col($select);

    foreach ($usermeta as $meta_key) {
        var_dump($meta_key);
    };
}

/**
*The second code example:
*/

function get_user_meta_key() {
    global $wpdb;

    $usermeta = $wpdb->get_col("SELECT distinct meta_key FROM $wpdb->usermeta" );

    foreach ($usermeta as $meta_key) {
        var_dump($meta_key);
    }
}