def extract_columns(semantics_dict):
    """Extracts the columns for each table (for the inner dictionary)
    from the semantic dictionary

    Args:
       semantics_dict (dict): dict of dicts

    Returns:
        (list): union of all columns from all tables

    """
    columns = set()
    for table in semantics_dict:
        column_dict = semantics_dict[table]
        columns = columns.union(set(column_dict))
    return list(columns)