How to use Column.isin with list?

According to documentation, isin takes a vararg, not a list. List is actually a confusing name here. You can try converting your List to vararg like this:

val items = List("a", "b", "c")

sqlContext.sql("select c1 from table")
          .filter($"c1".isin(items:_*))
          .collect
          .foreach(println)

Your variant with mkString compiles, because one single String is also a vararg (with number of arguments equal to 1), but it is proably not what you want to achieve.