Union[DataStream ->DataStream]
Union 算子主要是将两个或者多个输入的数据集合并成一个数据集，需要保证两个数据 集的格式一致，输出的数据集的格式和输入的数据集格式保持一致，如图所示，将灰色方块 数据集和黑色方块数据集合并成一个大的数据集

package transformation

import org.apache.flink.streaming.api.scala.StreamExecutionEnvironment

/**
 * @Author yqq
 * @Date 2021/12/26 16:07
 * @Version 1.0
 */
object Transformation {
  def main(args: Array[String]): Unit = {
    val en = StreamExecutionEnvironment.getExecutionEnvironment
    en.setParallelism(1)
    import org.apache.flink.streaming.api.scala._
    val stream1: DataStream[(String, Int)] = en.fromElements(("a", 1), ("b", 2))
    val stream2: DataStream[(String, Int)] = en.fromElements(("c", 3), ("d", 4))
    val result: DataStream[(String, Int)] = stream1.union(stream2)
    result.print()
    en.execute()
  }
}