จะรวมค่าแถวเดียวในฐานข้อมูล mysql จะใช้ code อย่างไร

<?php
if($period_id){
$sql = "SELECT sum(weight) FROM cf_ass_category where period_id='$period_id' ";
}else{
$sql = "SELECT sum(weight) FROM cf_ass_category order by period_id  ";
}
$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
$objDB = mysql_select_db("evaluation",$objConnect);
$strSQL = "SELECT * FROM cf_ass_category"; 
$objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
$array = mysql_fetch_array($objQuery);
$query= mysql_query($sql); 
$Body = mysql_result($query,0);
$Body_mean = $Body ;



 ?>