SQL中的LIKE不起作用

Question

I'm using a PDO to run this query, is there a way to see if the query return false? and why my query return nothing when I expect a result?

                $selectsql = "SELECT `firstname`,`surname`,`phone`,`address`,`username`,`StudentID` FROM `student_info` WHERE ? LIKE ?";
                $selectstmt = $conn->prepare($selectsql);
                $selectstmt->execute(array($searchby,"%{$searchval}%"));
                while($data = $selectstmt->fetch(PDO::FETCH_ASSOC)){
                  echo "
                  <tr>
                    <td>{$data['firstname']}</td>
                    <td>{$data['surname']}</td>
                    <td>{$data['phone']}</td>
                    <td>{$data['address']}</td>
                    <td>{$data['username']}</td>
                    <td>Delete Account</td>                    
                  </tr>
                  ";
                  var_dump($data);
                }   

I var dumpped the $searchby and the $searchval and they seems fine to me, no mistype or whatsoever.

Answer 1

The problem with your query is here :

$selectsql = "SELECT firstname`,`surname`,`phone`,`address`,`username`,`StudentID` FROM `student_info` WHERE ? LIKE ?";

That placeholder after where is messing up your query, after the where clause there should be a column name, by the way I see $searchby changes based on the item selected by the user. One thing you should note is that :

Table and Column names cannot be replaced by parameters in PDO.

Your query should look like.

<?php

$search = "%".$searchval."%";
$selectsql = "SELECT `firstname`,`surname`,`phone`,`address`,`username`,`StudentID` FROM `student_info` WHERE $searchby LIKE ?";
    $selectstmt = $conn->prepare($selectsql);
    $selectstmt->execute([$search]);
    $results = $selectstmt->fetch((PDO::FETCH_ASSOC));

    if(count($results) > 0){
      var_dump($results);
        foreach($results as $data):?>
          <tr>
              <td><?php echo $data['firstname'];?></td>
              <td><?php echo $data['surname'];?></td>
              <td><?php echo $data['phone'];?></td>
              <td><?php echo $data['address'];?></td>
              <td><?php echo $data['username'];?></td>
              <td>Delete Account</td>                    
        </tr>

      <?php
          endforeach;
    }else{

      echo "no results found";
    }
?>