Spark SQL：将嵌套的json类型DataFrame压平

参考：https://www.soinside.com/question/JjhZCytMUFpTNyk6W7ixZa

（没找到真正的出处，看拙劣的翻译，应该是从Stack Overflow扒过来的）

---

将数据如下的DataFrame压平

val json_string = """{
                   "Total Value": 3,
                   "Topic": "Example",
                   "values": [
                              {
                                "value1": "#example1",
                                "points": [
                                           [
                                           "123",
                                           "156"
                                          ]
                                    ],
                                "properties": {
                                 "date": "12-04-19",
                                 "model": "Model example 1"
                                    }
                                 },
                               {"value2": "#example2",
                                "points": [
                                           [
                                           "124",
                                           "157"
                                          ]
                                    ],
                                "properties": {
                                 "date": "12-05-19",
                                 "model": "Model example 2"
                                    }
                                 }
                              ]
                       }"""

希望得到如下输出

+-----------+-----------+----------+------------------+------------------+------------------------+-----------------------------+
|Total Value| Topic     |values 1 | values.points[0] | values.points[1] | values.properties.date | values.properties.model |
+-----------+-----------+----------+------------------+------------------+------------------------+-----------------------------+
| 3         | Example   | example1 | 123              | 156              | 12-04-19               |  Model Example 1         |
| 3         | Example   | example2 | 124              | 157              | 12-05-19               |    Model example 2         
+-----------+-----------+----------+------------------+------------------+------------------------+-----------------------------+

解决办法：

采用spark in-built函数，尤其是explode函数（此函数在org.apache.spark.sql.functions包下）

先将第一层“炸”开

scala> val df = spark.read.json(Seq(json_string).toDS)
scala> var dfd = df.select($"topic",$"total value",explode($"values").as("values"))

然后选择第二层中想要的列

scala> dfd.select($"topic",$"total value",$"values.points".getItem(0)(0).as("point_0"),$"values.points".getItem(0)(1).as("point_1"),$"values.properties.date".as("_date"),$"values.properties.model".as("_model")).show
+-------+-----------+-------+-------+--------+---------------+
|  topic|total value|point_0|point_1|   _date|         _model|
+-------+-----------+-------+-------+--------+---------------+
|Example|          3|    123|    156|12-04-19|Model example 1|
|Example|          3|    124|    157|12-05-19|Model example 2|
+-------+-----------+-------+-------+--------+---------------+

---

详细点的示例：

scala> val str = "{"avg_orders_count":[{"count":1.0,"days":3},{"count":0.6,"days":5},{"count":0.3,"days":10},{"count":0.2,"days":15},{"count":0.1,"days":30},{"count":0.066,"days":45},{"count":0.066,"days":60},{"count":0.053,"days":75},{"count":0.044,"days":90}],"m_hotel_id":"92500636"}"
str: String = {"avg_orders_count":[{"count":1.0,"days":3},{"count":0.6,"days":5},{"count":0.3,"days":10},{"count":0.2,"days":15},{"count":0.1,"days":30},{"count":0.066,"days":45},{"count":0.066,"days":60},{"count":0.053,"days":75},{"count":0.044,"days":90}],"m_hotel_id":"92500636"}

scala> val rdd = sc.makeRDD(str::Nil)
rdd: org.apache.spark.rdd.RDD[String] = ParallelCollectionRDD[3] at makeRDD at <console>:29


scala> val df = spark.read.json(rdd)

scala> df.show
+--------------------+----------+
|    avg_orders_count|m_hotel_id|
+--------------------+----------+
|[[1.0, 3], [0.6, ...|  92500636|
+--------------------+----------+

scala> df.select($"m_hotel_id",explode($"avg_orders_count")).show
+----------+-----------+
|m_hotel_id|        col|
+----------+-----------+
|  92500636|   [1.0, 3]|
|  92500636|   [0.6, 5]|
|  92500636|  [0.3, 10]|
|  92500636|  [0.2, 15]|
|  92500636|  [0.1, 30]|
|  92500636|[0.066, 45]|
|  92500636|[0.066, 60]|
|  92500636|[0.053, 75]|
|  92500636|[0.044, 90]|
+----------+-----------+

scala> val dfs = dfd.select($"m_hotel_id",$"ct.count", $"ct.days")
dfs: org.apache.spark.sql.DataFrame = [m_hotel_id: string, count: double ... 1 more field]

scala> dfs.show
+----------+-----+----+
|m_hotel_id|count|days|
+----------+-----+----+
|  92500636|  1.0|   3|
|  92500636|  0.6|   5|
|  92500636|  0.3|  10|
|  92500636|  0.2|  15|
|  92500636|  0.1|  30|
|  92500636|0.066|  45|
|  92500636|0.066|  60|
|  92500636|0.053|  75|
|  92500636|0.044|  90|
+----------+-----+----+