PHP, MYSQL no duplicates

Hi All,

I am using a mysql database that has a list of books. Columns: id, author, title, year, link(amazon).

So when I create a foreach loop to list out specific columns (such as all of the authors), it lists all of the authors, but if an author is written in the database more than once (because they've written more than one book) it echoes the author more than once as well. I would like to have each author name listed only once. I want it to remove duplicates Here is a snippet of my code.

<?php
function all_books(){

//(PDO database stuff);

 try {
        $data = $db->query("SELECT id, author, title, year, link FROM books ORDER BY author ASC");

    } catch (Exception $e) {
        echo "Data could not be retrieved from the database.";
        exit;
    }


    $books= $data->fetchAll(PDO::FETCH_ASSOC);    

    return $books; } 


} function headerNav($mainURL, $otherURL){

        $columns = all_books();
            echo "<dt><a href=\"../" . $mainURL . "\">" . $mainURL . "<a class='drop' href=''>+</a></dt><dd>";
            foreach($columns as $column){ ?>
                <li><a href="<?php echo "../results.php?s=" . urlencode($column["" . $otherURL . ""]); ?>"><?php echo $column["" . $otherURL . ""]; ?></li> <?php }?>
            <?php echo "</dd>";

    }       

This creates a simple navigation table, and without errors.

To extract what is important for my question, take the second half of it, specifically the:

<?php echo $column["" . $otherURL . ""]; ?> 

for example this could be:

<?php echo $column["author"]; ?>

And it would echo all of the authors. If the same author is listed twice, it will list that authors name twice. I would like for it to only list an author once for that situation.

I have tried using array_unique, but it does not recognize any of these items as arrays, but as strings. Mysql "DISTINCT" also has been giving me trouble, as I still want all of the information in my database (just filtered based on the situation).

Thanks everyone, and I hope I made sense.

Andrew