The following code is the result of me asking what I thought would be a simple question in the WordPress forums. I wanted to make a simple list of posts that shared two categories, but only those posts that were in both categories.

I take no credit for the code, I am simply replicating it here for storage/publicity purposes. The real gurus are alphaoide and amory.

I’m not using this anymore, but this is the exact code that I yoinked from my template:

<h2>Video Game Writing</h2>

<?php $my_posts = $wpdb->get_results( "
SELECT * FROM 
$wpdb->posts p1,
$wpdb->posts p2, 
$wpdb->post2cat c1,
$wpdb->post2cat c2 
WHERE 
p1.ID = p2.ID AND
p1.ID = c1.post_ID AND
p2.ID = c2.post_ID AND
c1.category_id = 2 AND
c2.category_id = 3
" );
?>
<?php if( $my_posts ) : foreach( $my_posts as $post ) : setup_postdata( $post ); ?>
<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
<?echo " - "; ?>
<?php the_date(); ?>
<?echo "<br />"; ?> 
<?php endforeach; endif; ?>

This would produce a simple list of posts that were in both categories 2 and 3 (for me, video games and writing). You can modify the category_id number to suit your needs, and the results can be customized to fit.

This was only tested in WP 1.5.x - I removed it before upgrading to 2.0. If anyone uses it in 2.0 and would like to leave a comment with its working status, that would be great and I’ll update the post, crediting you with the info.

Hope this helps someone else.

1. Project Dolphin Wordpress Plugin released
2. listing only posts that share two categories in WordPress
3. CSS styling for alternating comment colors with author differentiation in WordPress