how to change $sql when result is null or false

[gilgalbiblewhee]

if chapter or book returns false it needs to go to the next book from chapter 1. This is what I have so far:

$sql = "SELECT DISTINCT book_title FROM ".$dbTable3." WHERE book='".$getBook."' AND chapter='".$getChapter."' ORDER BY id ASC";
$result = mysql_query($sql) or die(mysql_error());
//echo $sql;

while($row = mysql_fetch_array($result)){
   $book_title = $row["book_title"];
} 

Book is number by the way and not a name as well as the chapter.