为什么在显示操作符后无法加入? [英] Why is join not possible after show operator?
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问题描述
以下代码工作正常,直到我在 agg
之后添加 show
.为什么 show
不可能?
The following code works fine until I add show
after agg
. Why is show
not possible?
val tempTableB = tableB.groupBy("idB")
.agg(first("numB").as("numB")) //when I add a .show here, it doesn't work
tableA.join(tempTableB, $"idA" === $"idB", "inner")
.drop("idA", "numA").show
错误说:
error: overloaded method value join with alternatives:
(right: org.apache.spark.sql.Dataset[_],joinExprs: org.apache.spark.sql.Column,joinType: String)org.apache.spark.sql.DataFrame <and>
(right: org.apache.spark.sql.Dataset[_],usingColumns: Seq[String],joinType: String)org.apache.spark.sql.DataFrame
cannot be applied to (Unit, org.apache.spark.sql.Column, String)
tableA.join(tempTableB, $"idA" === $"idB", "inner")
^
为什么会这样?
推荐答案
.show()
是一个函数,我们在 Scala 中称之为副作用.它打印到标准输出并返回 Unit()
,就像 println
.show()
is a function with, what we call in Scala, a side-effect. It prints to stdout and returns Unit()
, just like println
示例:
val a = Array(1,2,3).foreach(println)
a: Unit = ()
在scala中,你可以假设一切都是一个函数并且会返回一些东西.在您的情况下, Unit()
正在返回,这就是存储在 tempTableB
中的内容.
In scala, you can assume that everything is a function and will return something. In your case, Unit()
is being returned and that's what's getting stored in tempTableB
.
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