title: Spark: create a nested schema
body: <p>With spark,</p>
  
  <pre><code>import spark.implicits._
  val data = Seq(
    (1, ("value11", "value12")),
    (2, ("value21", "value22")),
    (3, ("value31", "value32"))
    )
  
   val df = data.toDF("id", "v1")
   df.printSchema()
  </code></pre>
  
  <p>The result is the following:</p>
  
  <pre><code>root
  |-- id: integer (nullable = false)
  |-- v1: struct (nullable = true)
  |    |-- _1: string (nullable = true)
  |    |-- _2: string (nullable = true)
  </code></pre>
  
  <p>Now if I want to create the schema myself, how should I process?</p>
  
  <pre><code>val schema = StructType(Array(
    StructField("id", IntegerType),
    StructField("nested", ???)
  ))
  </code></pre>
  
  <p>Thanks.</p>