カテゴリー中で何番目の記事か

JOIN が多くなるのでカテゴリーの slug よりも ID を使った方が良いかもしれませんが・・・。

functions.php

function post_number_in_cat($cat_slug) {
  global $wpdb, $post;
  $number = $wpdb->get_var("
    SELECT COUNT(posts.ID)
    FROM $wpdb->posts posts
    INNER JOIN $wpdb->term_relationships rels ON posts.ID = rels.object_id
    INNER JOIN $wpdb->term_taxonomy tax ON rels.term_taxonomy_id = tax.term_taxonomy_id
    INNER JOIN $wpdb->terms terms ON tax.term_id = terms.term_ID
    WHERE terms.slug = '{$cat_slug}'
    AND posts.post_date <= '{$post->post_date}'
    AND posts.post_status = 'publish'
    AND posts.post_type = 'post'
  ;");
  return $number;
}