How to use orderby() with descending order in Spark window functions?

问题

I need a window function that partitions by some keys (=column names), orders by another column name and returns the rows with top x ranks.

This works fine for ascending order:

def getTopX(df: DataFrame, top_x: String, top_key: String, top_value:String): DataFrame ={
    val top_keys: List[String] = top_key.split(", ").map(_.trim).toList
    val w = Window.partitionBy(top_keys(1),top_keys.drop(1):_*)
       .orderBy(top_value)
    val rankCondition = "rn < "+top_x.toString
    val dfTop = df.withColumn("rn",row_number().over(w))
      .where(rankCondition).drop("rn")
  return dfTop
}

But when I try to change it to orderBy(desc(top_value)) or orderBy(top_value.desc) in line 4, I get a syntax error. What's the correct syntax here?

回答1:

There are two versions of orderBy, one that works with strings and one that works with Column objects (API). Your code is using the first version, which does not allow for changing the sort order. You need to switch to the column version and then call the desc method, e.g., myCol.desc.

Now, we get into API design territory. The advantage of passing Column parameters is that you have a lot more flexibility, e.g., you can use expressions, etc. If you want to maintain an API that takes in a string as opposed to a Column, you need to convert the string to a column. There are a number of ways to do this and the easiest is to use org.apache.spark.sql.functions.col(myColName).

Putting it all together, we get

.orderBy(org.apache.spark.sql.functions.col(top_value).desc)

回答2:

Say for example, if we need to order by a column called Date in descending order in the Window function, use the $ symbol before the column name which will enable us to use the asc or desc syntax.

Window.orderBy($"Date".desc)

After specifying the column name in double quotes, give .desc which will sort in descending order.

回答3:

Column

col = new Column("ts")
col = col.desc()
WindowSpec w = Window.partitionBy("col1", "col2").orderBy(col)

来源：https://stackoverflow.com/questions/38572888/how-to-use-orderby-with-descending-order-in-spark-window-functions