Can't perform a UNION ALL MySQL query

[eluser]Ci beginner[/eluser]
[quote author="Colin Williams" date="1248684219"]You need to tell the DB class to not protect identifiers by supplying a third param of TRUE.

Code:

$this->db->where('UNION ALL "
        SELECT * FROM `blog` ORDER BY `id` DESC
        UNION ALL
        SELECT COUNT(*) FROM blog_comentarios, blog WHERE blog_comentarios.post_id = blog.id"', NULL, TRUE);

[/quote]

I can't try it now - I'm not @home. I trust you and it will probably works. Anyway, I don't fully understand why I need to use that. Would be nice if you can me a short explain about this, or if there's something about in the docs.

Thank you very much!