假设列名匹配在currency
列值:
import org.apache.spark.sql.functions.{lit, col, coalesce}
import org.apache.spark.sql.Column
// Dummy data
val df = sc.parallelize(Seq(
(49.5, "EUR", 99, 79, 69), (100.0, "GBP", 80, 120, 50)
)).toDF("paid", "currency", "EUR", "USD", "GBP")
// A list of available currencies
val currencies: List[String] = List("EUR", "USD", "GBP")
// Select listed value
val listedPrice: Column = coalesce(
currencies.map(c => when($"currency" === c, col(c)).otherwise(lit(null))): _*)
df.select($"*", (listedPrice - $"paid").alias("difference")).show
// +-----+--------+---+---+---+----------+
// | paid|currency|EUR|USD|GBP|difference|
// +-----+--------+---+---+---+----------+
// | 49.5| EUR| 99| 79| 69| 49.5|
// |100.0| GBP| 80|120| 50| -50.0|
// +-----+--------+---+---+---+----------+
与SQL等效的listedPrice
表达是这样的:
COALESCE(
CASE WHEN (currency = 'EUR') THEN EUR ELSE null,
CASE WHEN (currency = 'USD') THEN USD ELSE null,
CASE WHEN (currency = 'GBP') THEN GBP ELSE null
)
使用foldLeft
备选:
import org.apache.spark.sql.functions.when
val listedPriceViaFold = currencies.foldLeft(
lit(null))((acc, c) => when($"currency" === c, col(c)).otherwise(acc))
df.select($"*", (listedPriceViaFold - $"paid").alias("difference")).show
// +-----+--------+---+---+---+----------+
// | paid|currency|EUR|USD|GBP|difference|
// +-----+--------+---+---+---+----------+
// | 49.5| EUR| 99| 79| 69| 49.5|
// |100.0| GBP| 80|120| 50| -50.0|
// +-----+--------+---+---+---+----------+
其中listedPriceViaFold
转化为SQL语句:
CASE
WHEN (currency = 'GBP') THEN GBP
ELSE CASE
WHEN (currency = 'USD') THEN USD
ELSE CASE
WHEN (currency = 'EUR') THEN EUR
ELSE null
可惜我不知道任何内置的功能,这可以表达直接这样的SQL
CASE currency
WHEN 'EUR' THEN EUR
WHEN 'USD' THEN USD
WHEN 'GBP' THEN GBP
ELSE null
END
,但您可以用原始SQL使用此结构。
我的假设是不正确的,您可以简单地在列名称和currency
列中的值之间添加映射。
编辑:
另一种选择,这可能是,如果源支持谓词下推和高效柱修剪效率高,是由货币和工会子集:
currencies.map(
// for each currency filter and add difference
c => df.where($"currency" === c).withColumn("difference", $"paid" - col(c))
).reduce((df1, df2) => df1.unionAll(df2)) // Union
它相当于SQL LIKE这个:
SELECT *, EUR - paid AS difference FROM df WHERE currency = 'EUR'
UNION ALL
SELECT *, USD - paid AS difference FROM df WHERE currency = 'USD'
UNION ALL
SELECT *, GBP - paid AS difference FROM df WHERE currency = 'GBP'
我喜欢并且完美的coalesce()方法,很多Spark的工作,但它会很好!感谢那! – TomTom101
不客气。我已经添加了一个解决方案。 – zero323
第二个是相当好,聪明的方式来使用工会。 – mehmetminanc