$kampanya = mysql_query("SELECT * FROM substr(kampanya,0,250) as saydir ORDER BY id DESC");
while($row = mysql_fetch_array($kampanya))
{
echo $row["saydir"].' <a href="index.php?yer=kampanya&id='.$row["id"].'"> Devamı...</a>';
}

$kampanya = mysql_query("SELECT *, if(length(yazi)>250,rpad(rtrim(substr(yazi,1,252)),253,'..'),yazi) as yazi FROM kampanya ORDER BY id DESC");
while ($row = mysql_fetch_array($kampanya)) {           
     echo $row['yazi'];
    // ne istiyosan yazdır
}