def _build_constraint_str(constraint_l=None):
    """
    builds the contraint string expression for different
    queries. 
    Default is string 'true'.
    :param list constraint_l: list of constraints to be combined
    """
    if constraint_l:
        constraint_str = " && ".join(constraint_l)
    else:
        constraint_str = "true"
    return constraint_str