Will this PHP isUnique function work?

It has been a while since I have programmed PHP so I am very rusty, so I just want to double check my work. With the following code work as a way to check if the string is unique against a database?

        function isUnique($string, $type) {
        switch ($type) {
            case 'username':
                $query = $this->db->select('username')
                    ->from('olm_user')
                    ->where('username', $string);
                if (!$query->num_rows()) {
                    return false; // returns false if not taken?
                }
                break;
            case 'email':
                $query = $this->db->select('email')
                    ->from('olm_user')
                    ->where('email', $string);
                if (!$query->num_rows()) {
                    return false; // returns false if not taken?
              开发者_如何学运维  }
                break;
            case 'olname':
                $query = $this->db->select('olname')
                    ->from('olm_user')
                    ->where('olname', $string);
                if (!$query->num_rows()) {
                    return false; // returns false if not taken?
                }
                break;
        }
    }

If that's all you need to do (i.e. always query the olm_user table, and type always maps to the column name), maybe this would suffice:

<?php
function isUnique($string, $type) {
  $query = $this->db->select($type)
                    ->from('olm_user')
                    ->where($type, $string);
  return !$query->num_rows();
}