Wordpress，隐藏主页上的帖子

I have some automatic generated posts in my system, wp_posts table has additional field auto, by default this field is NULL, but for my specific posts it have some value. I don't wont to show such posts on main page, how can I hide them?

I think I need something like:

function exclude_post($query) {
    if ($query->is_home) {
        ...
    }

    return $query;
}

add_filter('pre_get_posts', 'exclude_post');

how can I add conditions like SQL "auto IS NOT NULL"?

If you were following other users suggestions and changed to post meta instead of an additional column in the posts table you could add:

$query->set( 'meta_query', array(
    array(
        'key' => 'meta_key_for_auto',
        'compare' => 'NOT EXISTS',
    ),
) );

If you're going to stick with the column in the posts table then pre_get_posts probably isn't the best hook to use. Instead look at posts_where. http://codex.wordpress.org/Plugin_API/Filter_Reference/posts_where