Spark获取DataFrame中列的方式：col；$；column；apply

       import spark.implicits._
       import org.apache.spark.sql.functions._
       import org.apache.spark.sql.Column

       df("columnName")            // On a specific DataFrame.
       col("columnName")           // A generic column no yet associated with a DataFrame.
       col("columnName.field")     // Extracting a struct field
       col("`a.column.with.dots`") // Escape `.` in column names.
       $"columnName"               // Scala short hand for a named column.
       expr("a + 1")               // A column that is constructed from a parsed SQL Expre