獲取兩個不同表格中兩列的差異？

Question

我的sql代碼執行，但我不知道如何得到purchase_request total_qty和purchase_order數量的差異。

表PURCHASE_ORDER

counter | qty |  
---------------   
100001 | 10 | 
100001 | 10 | 
100001 | 10 | 
100004 | 30 | 

表Purchase_Request

counter | total_qty | 
--------------------- 
100001 |  50 | 
100002 |  100 | 
100003 |  50 | 
100004 |  70 | 

我想代碼只是這樣的，但我不知道如何把它在我的代碼混合。

a.total_qty-b.qty as balance 

這是我的代碼

<?php 
    $mysqli = new mysqli("localhost", "root", "", "test"); 

     $result = $mysqli->query(" 
     select a.counter,a.total_qty from purchase_request a inner join purchase_order b on a.counter= b.counter group by a.counter 
     "); 
     echo'<table id="tfhover" cellspacing="0" class="tablesorter" style="text-transform:uppercase;" border="1px"> 
      <thead> 
      <tr> 
      <th></th> 
     <th>counter</th> 
     <th>QTY</th> 
     <th>balance</th> 
      </tr> 
      </thead>'; 
      echo'<tbody>'; 
     $i=1; 
    while($row = $result->fetch_assoc()){ 
     echo'<tr> 
       <td>'.$i++.'</td> 
       <td>'.$row['counter'].'</td> 
       <td>'.$row['total_qty'].'</td> 
       <td>'.$row['balance'].'</td> 
      </tr>'; 
      } 
     echo "</tbody></table>"; 

    ?> 

Answer 1

你試試這個？

select a.counter, 
      a.total_qty, 
      a.total_qty - b.qty balance 
     from (select counter, 
        sum(total_qty) total_qty 
       form purchase_request 
      group by counter) a 
inner join (select counter, 
        sum(qty) qty 
       from purchase_order 
      group by counter) b 
     on a.counter= b.counter 
    group by a.counter 

編輯：我知道了，你需要的是收集您的數量，然後做數學

Answer 2

select a.counter, 
      a.total_qty, 
      sum(a.total_qty) - sum(b.qty) as balance 
     from purchase_request a 
left inner join purchase_order b 
     on a.counter= b.counter 
    group by a.counter