php - How to group database tables using Group By -

i developing simple php shopping cart site connected databse named products , has tables orders,user,products. orders table has following fields : orderid , product_id , user_id , quantity ... when user orders example 3 products . data inserted session['cart'] below query

$query = "insert orders (product_id,user_id,quantity) values ('$key','$id','$value')";                  mysql_query($query); 

now problem if user orders more 1 product, 3 orderid's generated, because ordered 3 products , each orderid refers product. when use select statement shows orders of users want group them, if person ordered more 1 products @ time, list show me single orderid products user ordered it. below image of database table record orders table , select query using ..

orderid  product_id  user_id  quantity   

---

  1           4        1           1   3          21        1           3   4          19        1           1   5          21        1           3   6          19        1           2  $sql1=mysql_query("select * orders,user user.`user_id`=orders.`user_id`");       while($row = mysql_fetch_array($sql1))     {         $sreturn .='                 <tr>                 <td>' . $row['user_id'] . '</td>                 <td>' . $row['orderid'] . '</td>                 <td>' . $row['product_id'] . '</td>                 <td>' . $row['quantity'] . '</td>                 </tr>';     }    

what select query can used told me groupby sum ..

this u sum of products each user according product_id

updated

 select sum( orders.quantity ) , count( orders.order_id ) , orders.user_id      orders join user on (user.`user_id`=orders.`user_id`)      group orders.user_id