Modifying sql to pull posts from select categories

I’m using this code to pull the most commented posts from the entire database. I was wondering how I can edit it so that it only pulls posts from a select few categories and not the entire database.

<ul>
					<?php
						$sql = "
							SELECT *
							FROM $wpdb->posts
							WHERE 	post_date_gmt > ".(time()-strtotime('-2 weeks'))." AND post_type = 'post'
							ORDER BY comment_count DESC LIMIT 5
						";

						$top_posts = $wpdb->get_results($sql);

						foreach ($top_posts as $post)  {
							?>
							<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a><span><a class="comments" href="<?php the_permalink(); ?>#comments"><?php echo $post->comment_count; ?> Comments</a>Posted Under: <?php the_category(', '); ?></span></li>
							<?
						}
					?>
					</ul>