How to lower the case of column names of a data frame but not its values?

If you are using scala, you can simply do the following

import org.apache.spark.sql.functions._
df.select(df.columns.map(x => col(x).as(x.toLowerCase)): _*).show(false)

And if you are using pyspark, you can simply do the following

from pyspark.sql import functions as F
df.select([F.col(x).alias(x.lower()) for x in df.columns]).show()

How about this:

Some fake data:

scala> val df = spark.sql("select 'A' as AA, 'B' as BB")
df: org.apache.spark.sql.DataFrame = [AA: string, BB: string]

scala> df.show()
+---+---+
| AA| BB|
+---+---+
|  A|  B|
+---+---+

Now re-select all columns with a new name, which is just their lower-case version:

scala> val cols = df.columns.map(c => s"$c as ${c.toLowerCase}")
cols: Array[String] = Array(AA as aa, BB as bb)

scala> val lowerDf = df.selectExpr(cols:_*)
lowerDf: org.apache.spark.sql.DataFrame = [aa: string, bb: string]

scala> lowerDf.show()
+---+---+
| aa| bb|
+---+---+
|  A|  B|
+---+---+

Note: I use Scala. If you use PySpark and are not familiar with the Scala syntax, then df.columns.map(c => s"$c as ${c.toLowerCase}") is map(lambda c: c.lower(), df.columns) in Python and cols:_* becomes *cols. Please note I didn't run this translation.

Java 8 solution to convert the column names to lower case.

import static org.apache.spark.sql.functions.col;
import org.apache.spark.sql.Column;

df.select(Arrays.asList(df.columns()).stream().map(x -> col(x).as(x.toLowerCase())).toArray(size -> new Column[size])).show(false);