即使行尚未创建，数据库查询也会执行

Question

我不明白为什么在一个字段不存在的操作上mysqli_query会返回true。当字段run_id与变量$id匹配时，我正在尝试UPDATE一个字段。但是，如果未设置$id，则会执行if语句中的回显。会非常感谢你对这一现象的看法。

function sumMiles($id,$runnerRow,$weekArray,$cnn){
    //Calculate cumulative miles per week for every runner
    $weekMiles=0; //total miles per week
    foreach($weekArray as $weekday)
        {
            $weekMiles+=$runnerRow[$weekday];
        }
        $addMiles = "UPDATE run SET weekly_total = {$weekMiles} WHERE run_id=$id";//add weekly total into the database's run table 
        if (mysqli_query($cnn,$addMiles)){
            echo "Your weekly total has been updated";
            return $weekMiles;
        }
        else {echo "Your weekly total could not be updated";}
    }

我像这样调用这个函数：

$weekMiles = sumMiles($id,$dailyMileage,$weekArray,$cnn);

Answer 1

这一行不存在的事实不会在数据库中产生任何异常。

在运行更新之前，首先验证该行是否存在

  select count(1) from  run WHERE run_id=$id

编辑

需要检查Count(1)是否大于0

$query = "SELECT count(1) from run WHERE run_id=$id"; 
if (mysqli_query($cnn,$query)>0){ 
 $addMiles = "UPDATE run SET weekly_total = {$weekMiles} WHERE run_id=$id"; 
 echo "updated"; 
}