Spark获取DataFrame中列的方式--col，$，column，apply
1 官方说明
df("columnName") //在特定数据帧上
col("columnName") //尚未与数据帧关联的通用列
col("columnName.field") //提取结构字段
col("`a.column.with.dots`") //转义。在列名中
$"columnName" //Scala命名列的缩写
expr("a + 1") //由解析的SQL表达式构造的列
lit("abc") //产生文字（常量）值的列
2 使用时涉及到的的包
//
import spark.implicits._
import org.apache.spark.sql.functions._
import org.apache.spark.sql.Column
3 Demo
//
scala> val idCol = $"id"
idCol: org.apache.spark.sql.ColumnName = id
scala> val idCol = col("id")
idCol: org.apache.spark.sql.Column = id
scala> val idCol = column("id")
idCol: org.apache.spark.sql.Column = id
scala> val dataset = spark.range(5).toDF("text")
dataset: org.apache.spark.sql.DataFrame = [text: bigint]
scala> val textCol = dataset.col("text")
textCol: org.apache.spark.sql.Column = text
scala> val textCol = dataset.apply("text")
textCol: org.apache.spark.sql.Column = text
scala> val textCol = dataset("text")
textCol: org.apache.spark.sql.Column = text
