trying to use batch_update()

I am still having one more issue.
  $sql = "SELECT * FROM employees ORDER BY RAND() LIMIT " . $count;
        $data = array(
            'in_sample' => 1
        );
        $query = $this->db->query($sql);

        $this->db->set($data);
        $this->db->update('employees');

The query seems to work OK. When I take a look at the result(), I can see that 30 rows are chosen. But when update line runs, it is updating all the rows, not just the 30 that have been filtered. I can see that ALL the rows have in_sample set to one.  Anyone know what I am doing wrong in SQL? Additionally, does anyone know what the seed is for RAND()?

Perhaps I need to loop through the results and update them manually, one at a time?

proof that an old dog can learn new tricks