我最近遇到了一些类似的问题。让我在下面用你的案例来展示它们。
我正在创建两个具有相同数据的数据框
scala> val df_a = Seq((1, 2, "as"), (2,3,"ds"), (3,4,"ew"), (4, 1, "re"), (3,1,"ht")).toDF("a", "b", "c")
df_a: org.apache.spark.sql.DataFrame = [a: int, b: int ... 1 more field]
scala> val df_b = Seq((1, 2, "as"), (2,3,"ds"), (3,4,"ew"), (4, 1, "re"), (3,1,"ht")).toDF("a", "b", "c")
df_b: org.apache.spark.sql.DataFrame = [a: int, b: int ... 1 more field]
加入他们
scala> val df = df_a.join(df_b, df_a("b") === df_b("a"), "leftouter")
df: org.apache.spark.sql.DataFrame = [a: int, b: int ... 4 more fields]
scala> df.show
+---+---+---+---+---+---+
| a| b| c| a| b| c|
+---+---+---+---+---+---+
| 1| 2| as| 2| 3| ds|
| 2| 3| ds| 3| 1| ht|
| 2| 3| ds| 3| 4| ew|
| 3| 4| ew| 4| 1| re|
| 4| 1| re| 1| 2| as|
| 3| 1| ht| 1| 2| as|
+---+---+---+---+---+---+
让我们删除上述数据框中不存在的列
+---+---+---+---+---+---+
| a| b| c| a| b| c|
+---+---+---+---+---+---+
| 1| 2| as| 2| 3| ds|
| 2| 3| ds| 3| 1| ht|
| 2| 3| ds| 3| 4| ew|
| 3| 4| ew| 4| 1| re|
| 4| 1| re| 1| 2| as|
| 3| 1| ht| 1| 2| as|
+---+---+---+---+---+---+
理想情况下,我们希望 spark 会抛出错误,但它会成功执行。
现在,如果您从上述数据框中删除一列
scala> df.drop("a").show
+---+---+---+---+
| b| c| b| c|
+---+---+---+---+
| 2| as| 3| ds|
| 3| ds| 1| ht|
| 3| ds| 4| ew|
| 4| ew| 1| re|
| 1| re| 2| as|
| 1| ht| 2| as|
+---+---+---+---+
它会删除输入数据框中具有提供的列名的所有列。
如果要删除特定列,应按以下方式完成:
scala> df.drop(df_a("a")).show()
+---+---+---+---+---+
| b| c| a| b| c|
+---+---+---+---+---+
| 2| as| 2| 3| ds|
| 3| ds| 3| 1| ht|
| 3| ds| 3| 4| ew|
| 4| ew| 4| 1| re|
| 1| re| 1| 2| as|
| 1| ht| 1| 2| as|
+---+---+---+---+---+
我认为 spark 不接受您提供的输入(见下文):
scala> df.drop(df_a.a).show()
<console>:30: error: value a is not a member of org.apache.spark.sql.DataFrame
df.drop(df_a.a).show()
^
scala> df.drop(df_a."a").show()
<console>:1: error: identifier expected but string literal found.
df.drop(df_a."a").show()
^
如果您提供 drop 的输入,如下所示,它会执行但不会产生任何影响
scala> df.drop("df_a.a").show
+---+---+---+---+---+---+
| a| b| c| a| b| c|
+---+---+---+---+---+---+
| 1| 2| as| 2| 3| ds|
| 2| 3| ds| 3| 1| ht|
| 2| 3| ds| 3| 4| ew|
| 3| 4| ew| 4| 1| re|
| 4| 1| re| 1| 2| as|
| 3| 1| ht| 1| 2| as|
+---+---+---+---+---+---+
原因是,spark 将“df_a.a”解释为嵌套列。由于该列在理想情况下不存在,它应该引发错误,但如上所述,它只是执行。
希望这可以帮助..!!!