Can't perform a UNION ALL MySQL query

[eluser]Ci beginner[/eluser]
Hi there,

I want to perform a MySQL query with UNION ALL using active records. However, I'm unable to do it. I follow an example in the forum in a post from somewhere but didn't work. This is my code:

Code:

function index()
    {
        $data['title'] = "Blog Test";
        $data['head'] = "Head";
        $data['footer'] = "Footer";
        
        $this->db->select('blog_comentarios.post_id, blog.id');
        $this->db->from('blog_comentarios, blog');
        $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"');
        
        $data['query'] = $this->db->get();

        $this->load->view('vistablog', $data);

    }

And the error is:

Error Number: 1064

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'ALL " SELECT * FROM `blog` ORDER BY `id` DESC UNION ALL SELECT COUNT(*) FROM ' at line 3

SELECT `blog_comentarios`.`post_id`, `blog`.`id` FROM (`blog_comentarios`, `blog`) 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"

As you can see, is a syntax problem ("WHERE UNION ALL..."), but I don't really know how to make the UNION thing well. Could you give me an example?

Thank you very much in advance!