SELECT * FROM wp_posts
INNER JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
INNER JOIN wp_term_taxonomy ON (wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id)
WHERE (wp_term_taxonomy.term_id = 15
   AND wp_term_taxonomy.taxonomy = 'category'
   AND wp_posts.post_type = 'post'
   AND wp_posts.post_status = 'publish');