Question

我希望此代码在学生注册课程时向我显示错误消息

 $reds = mysqli_query($db,"SELECT count(*) FROM `course` " );
    $row = mysqli_fetch_row($reds);
    $num = $row[0];

echo $num>0 ; die();
   var_dump($num) ;die();
    if ($num>0){
   // echo "record already exits";
}

else {


    $sd = "INSERT INTO `users_course`(users_id,course_id) VALUES ( ' {$_SESSION['users_id']}', '$course_id')";
    $r = mysqli_query($db, $sd);
        if ($r) {

        $m = ' register';
        header('location: registercourse.php'); //redirect to index page after inserting

Answer 1

您只需在查询中传递users_id即可检查特定学生（用户ID）是否已注册该课程。

$reds = mysqli_query($db,"SELECT count(*) FROM `users_course` where users_id = `".$_SESSION['users_id']."` and course_id='".$course_id."'" );

因为使用当前的SELECT查询，您只检查课程表中是否存在任何课程。因为会有一些行，它会给出你的错误信息，即课程已经存在。

希望这是对你的明确解释。

Answer 2

我不知道您的数据库的结构是什么，但我认为您必须更改此查询＆＃39; SELECT count（）FROM course＆＃39; to＆＃34; SELECT count（）FROM users_course WHERE course_id =＆＃39; {$ _ SESSION [＆＃39; users_id＆＃39;]}＆＃39; AND user_id =＆＃39; $ course_id＆＃39;＆＃34;

Answer 3

如下所示： -

<?php
    session_start(); //start session to use session variable
    $user_id = $_SESSION['users_id'];
    $reds = mysqli_query($db,"SELECT * FROM `users_course` WHERE users_id = $user_id AND course_id= $course_id " );

    if (mysqli_num_rows($reds)>0){
        echo "record already exits";exit;
}else {
    $sd = "INSERT INTO `users_course`(users_id,course_id) VALUES ( $user_id, $course_id)";
    $r = mysqli_query($db, $sd);
    if ($r) {
        $m = ' register';
        header('location: registercourse.php'); //redirect to index page after inserting
    }else{
        echo "error:-".mysqli_error($db);
    }
}

注意： - 可能是当前为您工作的代码，但它对SQL注入是开放的，因此请尝试阅读prepared statements并使用它们

mysqli.prepare

请查看此示例： -

<?php
    session_start(); //start session to use session variable
    $user_id = $_SESSION['users_id'];
    $stmt = mysqli_prepare($db, "SELECT * FROM `users_course` WHERE users_id = ? AND course_id= ?");
    mysqli_stmt_bind_param($stmt, 'ii', $user_id, $course_id);

    /* execute query */
    mysqli_stmt_execute($stmt);

    /* store result */
    mysqli_stmt_store_result($stmt);

    if(mysqli_stmt_num_rows($stmt)>0){
        /* close statement */
        mysqli_stmt_close($stmt);
        echo "record already exits";exit;
    }else{
        $stmt = mysqli_prepare($db,"INSERT INTO `users_course`(users_id,course_id) VALUES (?, ?)";
        mysqli_stmt_bind_param($stmt, 'ii', $user_id, $course_id);
        if ( mysqli_stmt_execute($stmt)) {
            $m = ' register';
            header('location: registercourse.php'); //redirect to index page after inserting
        }else{
            echo "error:-".mysqli_error($db);
        }
    }

/* close connection */
mysqli_close($db);
?>

当课程已在php中注册时显示错误消息

3 个答案: