def flattenSchema(schema: StructType, prefix: String = null) : Array[Column] = {
  schema.fields.flatMap(f => {
    val colName = if (prefix == null) f.name else (prefix + "." + f.name)

    f.dataType match {
      case st: StructType => flattenSchema(st, colName)
      case _ => Array(col(colName).alias(colName))
    }
  })
}

var flattenedDf = json.select(flattenSchema(json.schema):_*)
display(flattenedDf)