def get_unique_values_in_column(df, col_name):
    """Get unique values in a column.
    
    Args:
        df (spark.DataFrame): Dataframe.
        col_name (str): Column name.
    
    Returns:
        spark.DataFrame: Unique values.
        
    **Examples**

        .. code-block:: python
        
            df = spark.createDataFrame([("a", 1), ("a", 2), ("c", 3)], ["letters", "numbers"])
            get_unique_values_in_column(df, "letters") 
            # [Row(letters='c'), Row(letters='a')]

    """
    return df.select(col_name).distinct().collect()