def make_unique_columns(df):
    """
    Renames the data frame columns to be unique when exported to an SQL
    database.

    Parameters
    ----------
    df : :class:`pandas.DataFrame`
        The data frame.
    """
    names = {}
    lnames = set()
    for x in list(df):
        n = x
        while n.lower() in lnames:
            n = n + '_'
        lnames.add(n.lower())
        names[x] = n
    df.rename(columns=names, inplace=True)
    return df
