How to get the last row from DataFrame?

问题

I hava a DataFrame,the DataFrame hava two column 'value' and 'timestamp',,the 'timestmp' is ordered,I want to get the last row of the DataFrame,what should I do?

this is my input:

+-----+---------+
|value|timestamp|
+-----+---------+
|    1|        1|
|    4|        2|
|    3|        3|
|    2|        4|
|    5|        5|
|    7|        6|
|    3|        7|
|    5|        8|
|    4|        9|
|   18|       10|
+-----+---------+

this is my code:

    val arr = Array((1,1),(4,2),(3,3),(2,4),(5,5),(7,6),(3,7),(5,8),(4,9),(18,10))
    var df=m_sparkCtx.parallelize(arr).toDF("value","timestamp")

this is my expected result:

+-----+---------+
|value|timestamp|
+-----+---------+
|   18|       10|
+-----+---------+

回答1:

I'd simply reduce:

df.reduce { (x, y) => 
  if (x.getAs[Int]("timestamp") > y.getAs[Int]("timestamp")) x else y 
}

回答2:

Try this, it works for me.

df.orderBy($"value".desc).show(1)

回答3:

I would use simply the query that - orders your table by descending order - takes 1st value from this order

df.createOrReplaceTempView("table_df")
query_latest_rec = """SELECT * FROM table_df ORDER BY value DESC limit 1"""
latest_rec = self.sqlContext.sql(query_latest_rec)
latest_rec.show()

回答4:

If your timestamp column is unique and is in increasing order then there are following ways to get the last row

println(df.sort($"timestamp", $"timestamp".desc).first())

// Output [1,1]

df.sort($"timestamp", $"timestamp".desc).take(1).foreach(println)

// Output [1,1]

df.where($"timestamp" === df.count()).show

Output:

+-----+---------+
|value|timestamp|
+-----+---------+
|   18|       10|
+-----+---------+

If not create a new column with the index and select the last index as below

val df1 = spark.sqlContext.createDataFrame(
    df.rdd.zipWithIndex.map {
  case (row, index) => Row.fromSeq(row.toSeq :+ index)
},
StructType(df.schema.fields :+ StructField("index", LongType, false)))

df1.where($"timestamp" === df.count()).drop("index").show

Output:

+-----+---------+
|value|timestamp|
+-----+---------+
|   18|       10|
+-----+---------+

回答5:

The most efficient way is to reduce your DataFrame. This gives you a single row which you can convert back to a DataFrame, but as it contains only 1 record, this does not make much sense.

sparkContext.parallelize(
  Seq(
  df.reduce {
    (a, b) => if (a.getAs[Int]("timestamp") > b.getAs[Int]("timestamp")) a else b 
   } match {case Row(value:Int,timestamp:Int) => (value,timestamp)}
  )
)
.toDF("value","timestamp")
.show


+-----+---------+
|value|timestamp|
+-----+---------+
|   18|       10|
+-----+---------+

Less efficient (as it needs shuffling) although shorter is this solution:

df
.where($"timestamp" === df.groupBy().agg(max($"timestamp")).map(_.getInt(0)).collect.head)

回答6:

Java:

Dataset<Row> sortDF = inputDF.orderBy(org.apache.spark.sql.functions.col(config.getIncrementingColumn()).desc());
Row row = sortDF.first()

回答7:

You can also use this function desc: Column desc(String columnName)

df.orderBy(desc("value")).show(1)

which gives same result as

df.orderBy($"value".desc).show(1)

来源：https://stackoverflow.com/questions/45406762/how-to-get-the-last-row-from-dataframe

标签

scala

apache-spark

apache-spark-sql

spark-dataframe