Show sticky posts on category archive pages

category-sticky-posts.php

/**
 * Adds sticky posts to your category archive
 * pages.
 *
 * Works in exactly the same way as sticky posts work 
 * on the homepage, including all the negatives like
 * breaking the posts_per_page and only showing
 * them on page 1 etc.
 * 
 * @param array $posts
 * @param object $wp_query
 * @author Tom Willmot - http://github.com/willmot/
 * @return array $posts
 */
function hm_category_sticky_posts( $posts, $wp_query ) {

	global $wp_the_query;
	
	// Don't continue if this isn't a category query, we're not in the main query or we're in the admin
	if ( ! $wp_query->is_category || $wp_query !== $wp_the_query || is_admin() )
		return $posts;

	global $wpdb;
	
	$q = $wp_query->query_vars;
	
	$page = absint( $q['paged'] );
	
	if ( empty( $page ) )
		$page = 1;
		
	$post_type = $q['post_type'];
	
	$sticky_posts = get_option( 'sticky_posts' );
	
	if ( $wp_query->is_category && $page <= 1 && is_array( $sticky_posts ) && !empty( $sticky_posts ) && ! $q['ignore_sticky_posts'] ) {
	   	
		$num_posts = count( $posts );
	   
	    $sticky_offset = 0;
	  
	    // Loop over posts and relocate stickies to the front.
	    for ( $i = 0; $i < $num_posts; $i++ ) {
	    	
	    	if ( in_array( $posts[$i]->ID, $sticky_posts ) ) {
	    	
	    		$sticky_post = $posts[$i];
	    	
	    		// Remove sticky from current position
	    		array_splice( $posts, $i, 1 );
	    	
	    		// Move to front, after other stickies
	    		array_splice( $posts, $sticky_offset, 0, array( $sticky_post ) );
	    	
	    		// Increment the sticky offset.  The next sticky will be placed at this offset.
	    		$sticky_offset++;
	    	
	    		// Remove post from sticky posts array
	    		$offset = array_search( $sticky_post->ID, $sticky_posts );
	    		unset( $sticky_posts[$offset] );
	    	
	    	}

	    }
	    
	    // If any posts have been excluded specifically, Ignore those that are sticky.
	    if ( !empty( $sticky_posts ) && !empty( $q['post__not_in'] ) )
	    	$sticky_posts = array_diff( $sticky_posts, $q['post__not_in'] );

	    // Fetch sticky posts that weren't in the query results
	    if ( !empty( $sticky_posts ) ) {
	    	
	    	$stickies__in = implode( ',', array_map( 'absint', $sticky_posts ));
	    	
	    	// honor post type(s) if not set to any
	    	$stickies_where = '';
	    	
	    	if ( 'any' != $post_type && '' != $post_type ) {
	    	
	    		if ( is_array( $post_type ) )
	    			$post_types = join( "', '", $post_type );
	    		
	    		else
	    			$post_types = $post_type;

	    		$stickies_where = "AND $wpdb->posts.post_type IN ('" . $post_types . "')";
	    	}

	    	$stickies = $wpdb->get_results( "SELECT wp_posts.* FROM $wpdb->posts INNER JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id) WHERE 1=1  AND ( wp_term_relationships.term_taxonomy_id IN (" . get_term( $wp_query->query_vars['cat'], 'category' )->term_taxonomy_id . ") ) AND $wpdb->posts.ID IN ($stickies__in) $stickies_where" );
			
	    	foreach ( $stickies as $sticky_post ) {
	    	
	    		// Ignore sticky posts are not published.
	    		if ( 'publish' != $sticky_post->post_status )
	    			continue;
	    	
	    		array_splice( $posts, $sticky_offset, 0, array( $sticky_post ) );
	    	
	    		$sticky_offset++;
	    	
	    	}
	    }
	}
	
	return $posts;

}
add_filter( 'the_posts', 'hm_category_sticky_posts', 10, 2 );

Just copy this into your themes functions.php file and your good to go.