Adding values from a table

[abrahamgarcia27]

I have a store number that i want to add and echo. This resembles an invoice so i my table has unit_price and total_value which is defined by by multiplying unit_price and pieces. Now that i want to print invoice i want to know how to add all items that are from that invoice_id

 

//get the total value for order
$sql_value = "SELECT total_value FROM `".TABLE_ORDERDETAILS."`WHERE order_id = $record_order[order_id]";
$rows_value = $db->fetch_all_array($sql_value);
foreach($rows_value as $record_value){
$value = $record_value[total_value];

}