如何使用selectExpr在Spark数据帧中强制转换结构数组?

时间:2018-07-31 19:00:38

标签: sql scala apache-spark dataframe apache-spark-sql

如何在spark数据帧中强制转换结构数组?

让我通过一个例子来说明我要做什么。 我们将从创建一个数据框开始,该数据框包含一个行数组和一个嵌套行。我的整数尚未在数据框中强制转换,而是创建为字符串:

import org.apache.spark.sql._
import org.apache.spark.sql.types._
val rows1 = Seq(
  Row("1", Row("a", "b"), "8.00", Seq(Row("1","2"), Row("12","22"))),
  Row("2", Row("c", "d"), "9.00", Seq(Row("3","4"), Row("33","44")))
)

val rows1Rdd = spark.sparkContext.parallelize(rows1, 4)

val schema1 = StructType(
  Seq(
    StructField("id", StringType, true),
    StructField("s1", StructType(
      Seq(
        StructField("x", StringType, true),
        StructField("y", StringType, true)
      )
    ), true),
    StructField("d", StringType, true),
    StructField("s2", ArrayType(StructType(
      Seq(
        StructField("u", StringType, true),
        StructField("v", StringType, true)
      )
    )), true)
  )
)

val df1 = spark.createDataFrame(rows1Rdd, schema1)

这是创建的数据框的架构:

       df1.printSchema
       root
       |-- id: string (nullable = true)
       |-- s1: struct (nullable = true)
       |    |-- x: string (nullable = true)
       |    |-- y: string (nullable = true)
       |-- d: string (nullable = true)
       |-- s2: array (nullable = true)
       |    |-- element: struct (containsNull = true)
       |    |    |-- u: string (nullable = true)
       |    |    |-- v: string (nullable = true)

我要做的是将所有可以为整数的字符串都转换为整数。我尝试执行以下操作,但没有成功:

df1.selectExpr("CAST (id AS INTEGER) as id",
  "STRUCT (s1.x, s1.y) AS s1",
  "CAST (d AS DECIMAL) as d",
  "Array (Struct(CAST (s2.u AS INTEGER), CAST (s2.v AS INTEGER))) as s2").show()

我遇到以下异常:

cannot resolve 'CAST(`s2`.`u` AS INT)' due to data type mismatch: cannot cast array<string> to int; line 1 pos 14;

任何人都有正确的查询将所有值转换为INTEGER吗?我将不胜感激。

非常感谢

1 个答案:

答案 0 :(得分:1)

您应该匹配完整的结构:

val result = df1.selectExpr(
  "CAST(id AS integer) id",
  "s1",
  "CAST(d AS decimal) d",
  "CAST(s2 AS array<struct<u:integer,v:integer>>) s2"
)

应该为您提供以下架构:

result.printSchema
root
 |-- id: integer (nullable = true)
 |-- s1: struct (nullable = true)
 |    |-- x: string (nullable = true)
 |    |-- y: string (nullable = true)
 |-- d: decimal(10,0) (nullable = true)
 |-- s2: array (nullable = true)
 |    |-- element: struct (containsNull = true)
 |    |    |-- u: integer (nullable = true)
 |    |    |-- v: integer (nullable = true)

和数据:

result.show
+---+-----+---+----------------+
| id|   s1|  d|              s2|
+---+-----+---+----------------+
|  1|[a,b]|  8|[[1,2], [12,22]]|
|  2|[c,d]|  9|[[3,4], [33,44]]|
+---+-----+---+----------------+