如何在不使用explode_outer的情况下扁平化PySpark 2.1.1模式来维护空数据点？

from pyspark.sql import SparkSession
from pyspark.sql.functions import col, coalesce

# 创建SparkSession
spark = SparkSession.builder.getOrCreate()

# 创建示例数据
data = [(1, "John", 25, None), (2, "Alice", None, "New York"), (3, "Bob", 30, "London")]
df = spark.createDataFrame(data, ["id", "name", "age", "city"])

# 扁平化模式并维护空数据点
flattened_df = df.select(
    col("id").alias("id"),
    col("name").alias("name"),
    coalesce(col("age"), col("age").cast("string")).alias("age"),
    coalesce(col("city"), col("city").cast("string")).alias("city")
).select(
    col("id"),
    col("name"),
    col("age"),
    col("city")
).alias("result")

# 显示结果
flattened_df.show()