$result = mysql_query( "select SUM(achievenum) as achieve_rows from games" );
$row = mysql_fetch_object( $result );
$total_achieve = $row->achieve_rows;

$result = mysql_query( "select SUM(achieveget) as get_rows from games" );
$row = mysql_fetch_object( $result );
$get_achieve = $row->get_rows;

$completed_rate = $get_achieve * 100 / $total_achieve;

Completion Rate : 32.3529411765%

32.3 %

select
    SUM(achievenum) as achieve_rows,
    select SUM(achievenum) as achieve_rows
from games

<?php
    echo round($completed_rate,1)."%";
?>

select ROUND(SUM(achieveget)*100/SUM(achievenum),1) as completion_rate from games