problema query con inner join

[LedGiallo]

salve, ho questa query:

SELECT post_categories.title as _title, users.username as _username, post.*, DATE_FORMAT(post.data_reg,'%d/%M/%Y ore %H:%i') as 'data_it' FROM users        INNER JOIN post 
          ON users.id = post.id_users INNER JOIN post_categories 
          ON post_categories.id = post.id_categories
          INNER JOIN tags ON tags.id = post_tags.id_tags
          WHERE post_tags.id_tags='".$tags_id."' and post.active=1 order by data_reg desc

che non funziona mi da errore:

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[42S22]: Column not found: 1054 Unknown column 'post_tags.id_tags' in 'where clause'' in /media/dati/www/site/lib/class-pager.php:33 Stack trace: #0 /media/dati/www/site/lib/class-pager.php(33): PDO->query('SELECT post_cat...') #1 /media/dati/www/site/tags.php(45): Pager->query('SELECT post_cat...', 100, '') #2 {main} thrown in/media/dati/www/site/lib/class-pager.php on line 33

queste sono le mie tabelle:

  CREATE TABLE `post_tags` (
  `id` bigint(11) NOT NULL auto_increment,
  `id_tags` bigint,
  `id_news` bigint,
  `data_reg` datetime,
  `_update` TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   PRIMARY KEY  (`id`))  ENGINE = MyISAM DEFAULT CHARSET=utf8;


  CREATE TABLE `tags` (
  `id` bigint(11) NOT NULL auto_increment,
  `name` varchar(255) unique,
  `data_reg` datetime,
  `_update` TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   PRIMARY KEY  (`id`))  ENGINE = MyISAM DEFAULT CHARSET=utf8;

il mio url è cosi:

http://localhost/site/tags.php?tags_id=1&tags_name=Video

e deve selezionarmi tutte le news che hanno il tag id 1 etc..

come si risolve il problema?

grazie mille.