我想添加一个新列，它是两个现有列的连接，我正在使用以下查询：这个查询有什么问题？我看到新列的“null”

df.select(df['DEST_COUNTRY_NAME'],df['ORIGIN_COUNTRY_NAME']).withColumn("COMPLETE_PATH",df['DEST_COUNTRY_NAME'] + ",").filter(df['DEST_COUNTRY_NAME']=='Egypt').show()

+-----------------+-------------------+-------------+
|DEST_COUNTRY_NAME|ORIGIN_COUNTRY_NAME|COMPLETE_PATH|
+-----------------+-------------------+-------------+
|            Egypt|      United States|         null|
|            Egypt|      United States|         null|
|            Egypt|      United States|         null|
|            Egypt|      United States|         null|
|            Egypt|      United States|         null|
|            Egypt|      United States|         null|
+-----------------+-------------------+-------------+