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php - COUNT query giving wrong results

问题描述

I have following table attendance

+--------------------------------+
| rollno | sub | date | presenty |
+--------------------------------+

It currently do not have any row. I want to add unique row with same value of rollno, sub, date and presenty. If any new entry comes with the same credentials, then I want to show user an alert message. Else, new entry should be created. I have written the following code:

$rollno = $_POST['rollno'];
$subject = $_POST['subject'];
$date = $_POST['date'];
$presenty = $_POST['presenty'];
$date_tmp = date_create($date);
$date_new = date_format($date_tmp, "d-m-Y");

$q = $conn->prepare("SELECT COUNT(`presenty`) FROM `attendance` WHERE `rollno` = ? AND `sub`=? AND `date`=?");
$q->bind_param("sss",$rollno,$subject,$date);
$q->execute();
$q->bind_result($pres);
$q->fetch();
$rows= $q->num_rows;
$q->close();
if($rows == 0){
    $insrtqry = $conn->prepare("INSERT INTO attendance(rollno, sub, date, presenty) VALUES(?,?,?,?)");
    $insrtqry->bind_param("ssss",$rollno, $subject, $date_new, $presenty);
    if($insrtqry->execute()){ ?>
        echo "Record Inserted Successfully";
    } else {
        echo "Record not added";
    }
} else {
    echo "Record already exists";
}

But, even if the table has no entry in it, when I try to insert a new record, the count query returns 1 and the else part executes. What is the problem? please help.

标签： phpmysql

解决方案

问题是你不能在这里使用 num_rows 因为 count 总是返回一些东西。因此，您将始终将 num_rows 设为 >0，因此对于计数，您应该使用 bind_result 方法。

mysqli_stmt::bind_result - mysqli_stmt_bind_result - 将变量绑定到准备好的语句以存储结果。将结果集中的列绑定到变量。当调用 fetch() 来获取数据时，MySQL 客户端/服务器协议将绑定列的数据放入指定的变量 var1, ....

      $rollno = $_POST['rollno'];
$subject = $_POST['subject'];
$date = $_POST['date'];
$date = date('Y-m-d',strtotime($date));
$presenty = $_POST['presenty'];

$q = $conn->prepare("SELECT COUNT(`presenty`) FROM `attendance` WHERE `rollno` = ? AND `sub`=? AND `date`=?");
$q->bind_param("sss",$rollno,$subject,$date);
$q->execute();
$q->bind_result($cnt);
$q->fetch();
$q->close();
if($cnt == 0){
    $insrtqry = $conn->prepare("INSERT INTO attendance(rollno, sub, date, presenty) VALUES(?,?,?,?)");
    $insrtqry->bind_param("ssss",$rollno, $subject, $date, $presenty);
    if($insrtqry->execute()){
        echo "Record Inserted Successfully";
    } else {
        echo "Record not added";
    }
} else {
    echo "Record already exists";
}

有关更多详细信息，您可以查看如何使用 bind_result 与 get_result 的示例他们有关于使用 get_result 的精彩详细信息

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