Reads the user labels for a list of clients.

def ReadAllClientLabels(self, cursor=None):
    """Reads the user labels for a list of clients."""

    cursor.execute("SELECT DISTINCT owner_username, label FROM client_labels")

    result = []
    for owner, label in cursor.fetchall():
      result.append(rdf_objects.ClientLabel(name=label, owner=owner))

    result.sort(key=lambda label: (label.owner, label.name))
    return result