如何使用应替换的空值(用 0)最大化每列? [英] How to max per column with nulls that should be replaced (with 0)?

问题描述

如何获得下面数据框中的 MAX?

How to get the MAX in the below dataframe?

val df_n = df.select($"ID").filter(($"READ") === "" && ($"ACT"!==""))

我必须找出ID的最大值，如果ID是NULL，我必须用0替换它.

I have to find out the MAX of ID and in case if ID is NULL, I have to replace it with 0.

推荐答案

以下内容如何?

scala> val df = Seq("0", null, "5", null, null, "-8").toDF("id")
df: org.apache.spark.sql.DataFrame = [id: string]

scala> df.printSchema
root
 |-- id: string (nullable = true)

scala> df.withColumn("idAsLong", $"id" cast "long").printSchema
root
 |-- id: string (nullable = true)
 |-- idAsLong: long (nullable = true)


scala> val testDF = df.withColumn("idAsLong", $"id" cast "long")
testDF: org.apache.spark.sql.DataFrame = [id: string, idAsLong: bigint]

scala> testDF.show
+----+--------+
|  id|idAsLong|
+----+--------+
|   0|       0|
|null|    null|
|   5|       5|
|null|    null|
|null|    null|
|  -8|      -8|
+----+--------+

解决方案

scala> testDF.agg(max("idAsLong")).show
+-------------+
|max(idAsLong)|
+-------------+
|            5|
+-------------+

使用 na 运算符

如果只有负值和 null 并且 null 是最大值怎么办?在 Dataset 上使用 na 运算符.

Using na Operator

What if you had only negative values and null and so null is the maximum value? Use na operator on Dataset.

val withNulls = Seq("-1", "-5", null, null, "-333", null)
  .toDF("id")
  .withColumn("asInt", $"id" cast "int")  // <-- column of type int with nulls

scala> withNulls.na.fill(Map("asInt" -> 0)).agg(max("asInt")).show
+----------+
|max(asInt)|
+----------+
|         0|
+----------+

如果没有 na 和替换 null，它根本就行不通.

Without na and replacing null it simply won't work.

scala> withNulls.agg(max("asInt")).show
+----------+
|max(asInt)|
+----------+
|        -1|
+----------+

见na: DataFrameNaFunctions.

这篇关于如何使用应替换的空值(用 0)最大化每列?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！