RDD，DataFrame与DataSet的相互转换

Dataframe相当于在RDD的基础上添加了结构，DataSet相当于在Dataframe的基础上添加了类型

// 创建rdd
scala> val rdd = sc.makeRDD(List(("zhangsan",18)))
rdd: org.apache.spark.rdd.RDD[(String, Int)] = ParallelCollectionRDD[23] at makeRDD at :28

scala> rdd.collect().foreach(println)
(zhangsan,18)

// rdd->df rdd.toDF(列名1,列名2,...)
scala> val df = rdd.toDF("name","age")
df: org.apache.spark.sql.Dataframe = [name: string, age: int]

// 打印输出
scala> df.show
+--------+---+
|    name|age|
+--------+---+
|zhangsan| 18|
+--------+---+

// rdd->ds 样例类RDD.toDS()
// 创建样例类
case class User(name: String, age: Int)

scala> val rdd1 = rdd.map(data=>User(data._1,data._2))
rdd1: org.apache.spark.rdd.RDD[User] = MapPartitionsRDD[27] at map at :31

scala> val ds = rdd1.toDS()
ds: org.apache.spark.sql.Dataset[User] = [name: string, age: int]

scala> ds.show
+--------+---+
|    name|age|
+--------+---+
|zhangsan| 18|
+--------+---+

// df -> RDD df.rdd
scala> val dfToRDD = df.rdd
dfToRDD: org.apache.spark.rdd.RDD[org.apache.spark.sql.Row] = MapPartitionsRDD[35] at rdd at :29

scala> dfToRDD.collect().foreach(println)
[zhangsan,18]

// df -> ds ds.as[样例类]
scala> val dfTods = df.as[User]
dfTods: org.apache.spark.sql.Dataset[User] = [name: string, age: int]

scala> dfTods.show
+--------+---+
|    name|age|
+--------+---+
|zhangsan| 18|
+--------+---+

// ds -> RDD ds.rdd
scala> val dsToRDD = ds.rdd
dsToRDD: org.apache.spark.rdd.RDD[User] = MapPartitionsRDD[44] at rdd at :29

scala> dsToRDD.collect().foreach(println)
User(zhangsan,18)

// ds -> df ds.toDF
scala> val dsTodf = ds.toDF
dsTodf: org.apache.spark.sql.Dataframe = [name: string, age: int]

scala> dsTodf.show
+--------+---+
|    name|age|
+--------+---+
|zhangsan| 18|
+--------+---+