如何将整个pyspark数据帧的大小写更改为下或上

from pyspark.sql import SparkSession
from pyspark.sql.functions import lower, upper

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

# 创建示例数据帧
data = [("John", "Doe"), ("Alice", "Smith"), ("Bob", "Johnson")]
df = spark.createDataFrame(data, ["first_name", "last_name"])

# 将first_name和last_name列的值转换为小写
df_lower = df.withColumn("first_name_lower", lower(df["first_name"])) \
             .withColumn("last_name_lower", lower(df["last_name"]))

# 将first_name和last_name列的值转换为大写
df_upper = df.withColumn("first_name_upper", upper(df["first_name"])) \
             .withColumn("last_name_upper", upper(df["last_name"]))

# 显示结果
df_lower.show()
df_upper.show()