mysql_fetch_assoc为什么只显示一组数据

<?php
$link = mysql_connect('127.0.0.1','root','root') or die('数据库链接失败');
mysql_select_db('test');
mysql_query('set name "utf8"');
$result = mysql_query('select * from users');
print_r($result);
echo '<br>';
$row = mysql_fetch_assoc($result);
print_r($row);
?>输出:
Resource id #3
Array
(
[id] => 1
[username] => tom
[pid] => 3
)