Why is join not possible after show operator?

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.show() is a function with, what we call in Scala, a side-effect. It prints to stdout and returns Unit(), just like println

Example: 

val a  = Array(1,2,3).foreach(println)
a: Unit = ()

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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