where i went wrong please help urgent

<?php include 'dbConfig.php';
 ?>
        <div class="table" style="display: <?php echo isset($_POST['find']) ? "none" : "block" ?>">

        <table class="table table-bordered table-striped msg-reports-table" id="table" display="">
        <tr>
          
        </tr> 
        <?php
      //$res=mysqli_query($db,"Select f.fname,f.sname,f.mail,f.num,f.gen,s.tadd,s.padd,s.amail,s.anum,s.status from first f,second s ");
//echo $$sql1="select f.fname,f.sname,f.mail,f.num,f.gen,s.tadd,s.padd,s.amail,s.anum,s.status from 
// ( SELECT 
// * first from to_numbers
//  ) f left outer join
//   ( SELECT 
// *  from second  
//    ) s";res;

//      
$res=   mysqli_query($db,"Select * from first UNION select * from second ");
        echo "<tr><td colspan='5'></td></tr>";
             while($row = mysqli_fetch_assoc($res))  
                          {  
                               echo '  
                               <tr>  
                                    <td>'.$row["f.fname"].'</td>  
                                    <td>'.$row["f.sname"].'</td>  
                                    <td>'.$row["f.mail"].'</td>  
                                    <td>'.$row["f.num"].'</td>  
                                    <td>'.$row["f.gen"].'</td>  
                                    <td>'.$row["s.tadd"].'</td>  
                                    <td>'.$row["s.padd"].'</td>  
                                    <td>'.$row["s.amail"].'</td>  
                                    <td>'.$row["s.anum"].'</td>  
                                    <td>'.$row["s.status"].'</td>  
                                     
                                     
                               </tr>  
                               ';  
                          }  
  
  ?>
  </table>
</div>

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 Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, boolean given in C:\xampp\htdocs\third.php on line 22