def organize_columns(columns):
    """Locates data column keys and their respective mean data keys.

    Args:
        columns: iterable of dataframe column names

    Returns:
        keywords: iterable of tuples containing (data key, data mean key | '')
    """

    located = []
    keywords = []
    for kw in sorted(columns):
        if kw in located:
            continue
        mean_keys = [mkey for mkey in columns if mkey.startswith(kw+'_MEAN')]
        if not mean_keys:
            keywords.append((kw, ''))
            continue
        for mean_key in mean_keys:
            keywords.append((kw, mean_key))
            located.append(mean_key)

    return keywords