Php 使用自然联接运行查询时出现问题

我这样做是否正确？每次运行查询时，我都会得到NULL

 $con = mysqli_connect("localhost:3307","root","");

    mysqli_select_db($con, 'project');

    // Check connection
    if (!$con){
      echo "Failed to connect to MySQL: " . mysqli_connect_error();
    };

    mysqli_query ($con,"set character_set_results='utf8'");
$inv_id= "Please enter an invoice ID";
$cus_name = "";
$cus_phone = "";
$inv_total = "" ;
$ship_date = "";

if(isset($_POST['submit'])){
    $inv_id = $_POST['inv_id'];
    //echo $inv_id;
    $sql = 'SELECT inv_total, ship_date, cus_name, cus_phone
            from invoiceheader natural join shipment natural join customer
            where inv_id = \'' . $inv_id . '\' '; 
            //echo $sql;

    $result = mysqli_query($con, $sql);
        if(!$result)
           {
                echo "SQL Select query failed!";
        } 
        $row = $result->fetch_assoc();  
        //var_dump($row);   
        $cus_name = $row["cus_name"];
        $cus_phone = $row["cus_phone"];
        $inv_total = $row["inv_total"];
        $ship_date = $row["ship_date"];
}

?>

我在var_dump上得到null，在sql命令行上得到空集，但没有错误

mysql>从invoiceheader中选择*；
+--------+---------------------+--------+-----------+---------------------+
|库存标识|库存日期|客户标识|库存总额|上次修改日期||
+--------+---------------------+--------+-----------+---------------------+
| 111111 | 2016-11-30 23:35:22 |      1 |        53 | 2016-11-30 23:35:22 |
| 111112 | 2016-11-30 23:35:22 |      2 |      41.5 | 2016-11-30 23:35:22 |
| 111113 | 2016-11-30 23:35:22 |      3 |        26 | 2016-11-30 23:35:22 |
| 111114 | 2016-11-30 23:35:22 |      4 |        51 | 2016-11-30 23:35:22 |
| 111115 | 2016-11-30 23:35:22 |      5 |     38.75 | 2016-11-30 23:35:22 |
| 111116 | 2016-11-30 23:35:22 |      6 |        55 | 2016-11-30 23:35:22 |
| 111117 | 2016-11-30 23:35:22 |      7 |     41.25 | 2016-11-30 23:35:22 |
| 111118 | 2016-11-30 23:35:22 |      8 |     71.25 | 2016-11-30 23:35:22 |
| 111119 | 2016-11-30 23:35:22 |      9 |     28.25 | 2016-11-30 23:35:22 |
| 111120 | 2016-11-30 23:35:22 |     10 |     69.25 | 2016-11-30 23:35:22 |
| 111121 | 2016-11-30 23:35:22 |      6 |      65.5 | 2016-11-30 23:35:22 |
| 111122 | 2016-11-30 23:35:22 |      3 |        58 | 2016-11-30 23:35:22 |
+--------+---------------------+--------+-----------+---------------------+
一组12行（0.00秒）
mysql>从客户中选择*；
+--------+--------------------+------------+-----------------+--------------------+---------------+-------+-------+---------------+---------------------+---------------------+
|客户id |客户姓名|客户电话|客户电子邮件|街道|城市|州|邮编|最后订单日期|创建日期|最后修改日期||
+--------+--------------------+------------+-----------------+--------------------+---------------+-------+-------+---------------+---------------------+---------------------+
|1 |约翰·默里| 1111111 |cus1@email.com|第一街123 |伊根|明尼苏达| 51224 | 2017-01-10 | 2017-04-05 19:16:07 | 2017-04-05 19:16:07|
|2 | Rachael Smith | 2222 |cus2@email.com|第二街123号|明尼苏达州明尼通卡市| 55266 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|3 |艾迪生·布朗| 3333333 |cus3@email.com| 123第三街|布卢明顿|明尼苏达州| 55757 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|4 |迈克·刘易斯| 4444 |cus4@email.com|第四街123号|埃根|明尼苏达| 51224 | 2017-01-10 | 2017-04-05 19:16:08 | 2017-04-05 19:16:08|
|5 |马特·柯林斯| 5555 |cus5@email.com|第五街123号|新希望| MN | 55336 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|6 |乔纳森·埃利奥特| 6666 |cus6@email.com|第六街123号|水城|明尼苏达| 50221 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|7 | Jessica Stratmeyer | 7777 |cus7@email.com|第七街123号|埃根|明尼苏达| 51224 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|8 |大卫·威尔逊| 8888888888 |cus8@email.com|第八街123号|明尼苏达州明尼阿波利斯市| 55346 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|9 |麦克林·约翰逊| 9999999 |cus9@email.com|第九街123号|圣路易斯公园|明尼苏达州| 55348 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
|10 | Tony Bui | 1230 |cus10@email.com|第十街123号| Chanhassen | MN | 55341 | 2017-01-10 | 2016-12-30 23:35:22 | 2016-12-30 23:35:22|
+--------+--------------------+------------+-----------------+--------------------+---------------+-------+-------+---------------+---------------------+---------------------+
一组10行（0.00秒）
mysql>从发货中选择*；
+---------+--------+--------+------------+---------------------+---------------------+
|发货id |库存id |客户id |发货日期|创建日期|上次修改日期||
+---------+--------+--------+------------+---------------------+---------------------+
|      21 | 111111 |      1 | 2016-12-01 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      22 | 111112 |      2 | 2016-12-03 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      23 | 111113 |      3 | 2016-12-01 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      24 | 111114 |      4 | 2016-12-05 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      25 | 111115 |      5 | 2016-12-01 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      26 | 111116 |      6 | 2016-12-02 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      27 | 111117 |      7 | 2016-12-01 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      28 | 111118 |      8 | 2016-12-03 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      29 | 111119 |      9 | 2016-12-05 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      30 | 111120 |     10 | 2016-12-02 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      31 | 111121 |      6 | 2016-12-04 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
|      32 | 111122 |      3 | 2016-12-01 | 2016-11-30 23:35:22 | 2016-11-30 23:35:22 |
+---------+--------+--------+------------+---------------------+---------------------+
一组12行（0.00秒）
mysql>选择客户名称、客户电话、库存id、库存总额、发货日期
->来自客户自然加入发货自然加入发票抬头
->其中，inv_id=111111；
空集（0.00秒）

回应您的评论：

通过隔离执行的查询，将数据库逻辑与应用程序逻辑分开：在应用程序中打印它或自己重新创建它

在MySQL控制台中运行查询，或通过phpMyAdmin或用于此操作的任何软件运行查询

它的回报是什么？如果查询