php中用多列和连接2个表

我试图用SUM连接PHP中MYSQL的2个表。但当我得到结果时，它会显示乘法结果。我不知道我的剧本出了什么问题。请帮忙。我的脚本如下表所示：

   Table1: purchase
   id      model       quantity     retquantity   
   1        D54          10            NULL
   2        D54         NULL            5
   3        M90          10            NULL
   4        M95          10            NULL
   5        M90           5

  Table2: sale

  id      model       quantity      retquantity
   1        D54          5            NULL
   2        D54         NULL            5
   3        M90         NULL            5
   4        M95           6            NULL
   5        M90          10            Null

我的php脚本：

    <?php
        $count=0;
        $sql = "SELECT purchase.model,sale.model, 
    SUM(purchase.quantity) AS pquantity,SUM(sale.quantity) AS squantity,
    SUM(purchase.retquantity) AS retpquantity,
     SUM(sale.retquantity) AS sretquantity
    FROM (select model,
    sum(quantity) AS pquantity,sum(retquantity) AS retpquantity from 
     purchase group by model)purchase
    INNER JOIN (select model,
     sum(quantity) AS squantity,
    sum(retquantity) AS retsquantity
     from sale group by model)sale
     ON purchase.model=sale.model
    GROUP BY purchase.model,sale.model";

    $result = $cn->query($sql);
    if ($result->num_rows > 0) {

    while($row = $result->fetch_assoc()) {

    ?>
    <td><?php echo ++$count; ?></td>
                    <td><?php echo $row['model']; ?></td>
                    <td><?php echo $row['pquantity']; ?></td>
                    <td><?php echo $row['squantity']; ?></td>
                    <td><?php echo $row['retpquantity']; ?></td>
                    <td><?php echo $row['retsquantity']; ?></td>

                    <?php }} $cn->close();?>

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我没有得到任何结果。请帮忙

尝试此查询可能会有所帮助

SELECT id, SUM(model) as model, SUM(quantity) as quantity, SUM(retquantity) as retquantity FROM (
       SELECT * FROM purchase
       UNION ALL
       SELECT * FROM sale
       UNION ALL
       ) AS somealias GROUP BY id

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您的mysql查询在哪里？您会收到错误或其他任何信息…首先打印您的查询您正在执行的内容以清除查询有效性，并共享您的查询..这是查询$sql=SELECT PUSHARE.model，sal…..我收到此错误通知：尝试获取非对象上成员函数fetch_assoc（）的非对象调用属性