Automatically assign posts by author to a specific category

The solution is quite simple. And is subdivided into two separate processes – querying the posts and modifying their category terms.

Querying the posts

get_posts is a function that returns posts. A full list of arguments can be seen in WP_Query parameters

To get all posts for a specific author, you do something like:

get_posts( 'numberposts=1&author_name=john' );

Loop over each entry like so:

foreach ( get_posts( 'author_name=john' ) as $post ) {
    // set categories...
}

Setting category terms

wp_set_object_terms is the function you’ll be looking at and using.

wp_set_object_terms( $post->ID, 'John's Category', 'category', false );

Putting it together

I would probably go for something like this:

global $wpdb; // get the global db object

foreach ( (array) $wpdb->get_results("SELECT DISTINCT post_author FROM $wpdb->posts WHERE post_type="post" GROUP BY post_author") as $row ) {

    $author = get_userdata( $row->post_author ); // get all author data

    foreach ( get_posts( 'numberposts=-1&author=".$row->post_author ) as $post ) {
        switch ( $author->username ) {
            case "joe':
                $category = 'Joe's Entries';
            ...
        }

        // set categories
        wp_set_object_terms( $post->ID, $category, 'category', false );
    }
}

I’m sure there are ways in which you can further optimize this. Run this once to fix things. Then attach to save_post with just the set category part for future.