مشكلة في php رسالة خطأ

الخطأ الدي يظهر :

Warning: mysql_fetch_assoc() expects parameter 1 to be resource, boolean given in D:\AppServ\www\testforeach\mune.php on line 8

الكود في المحرر:

function print_books($sql = ''){

$res = mysql_query($sql);
$i=0;
while ($book = mysql_fetch_assoc($res) ) {

 ?>

<div class="col-lg-6 col-md-6 ">
<img style="max-width: 100px;height: auto; "
       src="<?php echo $book['image']; ?>">

  <h2><?php echo $book['title']; ?></h2>

  <h4>Author:  <?php echo $book['name']; ?></h4>
  <p>
  <?php echo $book['brief']; ?>
  </p>

  <button class="btn btn-primary"><?php echo $book['price']; ?></button>

</div>


  <?php
     $i++;
  if($i%2 == 0){
    echo"<div class='clearfix'></div>";
  }