WordPressPostCategoriesExport

//snip from wordpress/wp-admin/includes/export.php

/*$where = '';
if ( $author and $author != 'all' ) {
	$author_id = (int) $author;
	$where = $wpdb->prepare(" WHERE post_author = %d ", $author_id);
}*/

$query = "
	SELECT ID
	FROM $wpdb->posts as wpost
	INNER JOIN $wpdb->term_relationships
	ON (wpost.ID = $wpdb->term_relationships.object_id)
	INNER JOIN $wpdb->term_taxonomy
	ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
	AND $wpdb->term_taxonomy.taxonomy = 'category'
	AND $wpdb->term_taxonomy.term_id IN (3,13,7,257)
	AND wpost.post_status ='publish' and wpost.post_type='post';
";

//$wpdb->term_taxonomy.term_id IN (3,13,7,257) selects just the given category IDs

// grab a snapshot of post IDs, just in case it changes during the export
//$post_ids = $wpdb->get_col("SELECT wp_posts.ID FROM wp_posts left join wp_term_relationships on wp_posts.ID=wp_term_relationships.object_id $where ORDER BY post_date_gmt ASC");
$post_ids = $wpdb->get_col($query);