SparkSql系列(7/25) 去重方法

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消除重复的数据可以通过使用 distinct 和 dropDuplicates 两个方法，二者的区别在于，distinct 是所有的列进行去重的操作，假如你的 DataFrame里面有10列，那么只有这10列完全相同才会去重，dropDuplicates 则是可以指定列进行去重，相当于是distinct 的缩小版

构建 DataFrame

import spark.implicits._

val simpleData = Seq(("James", "Sales", 3000),
  ("Michael", "Sales", 4600),
  ("Robert", "Sales", 4100),
  ("Maria", "Finance", 3000),
  ("James", "Sales", 3000),
  ("Scott", "Finance", 3300),
  ("Jen", "Finance", 3900),
  ("Jeff", "Marketing", 3000),
  ("Kumar", "Marketing", 2000),
  ("Saif", "Sales", 4100)
)
val df = simpleData.toDF("employee_name", "department", "salary")
df.show()

输出如下所示：

+-------------+----------+------+
|employee_name|department|salary|
+-------------+----------+------+
|        James|     Sales|  3000|
|      Michael|     Sales|  4600|
|       Robert|     Sales|  4100|
|        Maria|   Finance|  3000|
|        James|     Sales|  3000|
|        Scott|   Finance|  3300|
|          Jen|   Finance|  3900|
|         Jeff| Marketing|  3000|
|        Kumar| Marketing|  2000|
|         Saif|     Sales|  4100|
+-------------+----------+------+

1. 使用 distinct 去重

//Distinct all columns
val distinctDF = df.distinct()
println("Distinct count: "+distinctDF.count())
distinctDF.show(false)

distinct() 函数会返回一个新的 DataFrame ，而不是原地修改。

Distinct count: 9
+-------------+----------+------+
|employee_name|department|salary|
+-------------+----------+------+
|James        |Sales     |3000  |
|Michael      |Sales     |4600  |
|Maria        |Finance   |3000  |
|Robert       |Sales     |4100  |
|Saif         |Sales     |4100  |
|Scott        |Finance   |3300  |
|Jeff         |Marketing |3000  |
|Jen          |Finance   |3900  |
|Kumar        |Marketing |2000  |
+-------------+----------+------+

或者你不指定 dropDuplicates 的参数，也是可以达到上面的效果

val df2 = df.dropDuplicates()
println("Distinct count: "+df2.count())
df2.show(false)

2. 使用 dropDuplicates 按照指定列进行去重

与 distinct 一样，函数都会返回一个新的 DataFrame

//Distinct using dropDuplicates
val dropDisDF = df.dropDuplicates("department","salary")
println("Distinct count of department & salary : "+dropDisDF.count())
dropDisDF.show(false)

结果如下所示：

Distinct count of department & salary : 8
+-------------+----------+------+
|employee_name|department|salary|
+-------------+----------+------+
|Jen          |Finance   |3900  |
|Maria        |Finance   |3000  |
|Scott        |Finance   |3300  |
|Michael      |Sales     |4600  |
|Kumar        |Marketing |2000  |
|Robert       |Sales     |4100  |
|James        |Sales     |3000  |
|Jeff         |Marketing |3000  |
+-------------+----------+------+

3. 完整的示例代码

import org.apache.spark.sql.SparkSession
import org.apache.spark.sql.functions._

object SQLDistinct extends App {

  val spark: SparkSession = SparkSession.builder()
    .master("local[1]")
    .appName("SparkByExamples.com")
    .getOrCreate()

  spark.sparkContext.setLogLevel("ERROR")

  import spark.implicits._

  val simpleData = Seq(("James", "Sales", 3000),
    ("Michael", "Sales", 4600),
    ("Robert", "Sales", 4100),
    ("Maria", "Finance", 3000),
    ("James", "Sales", 3000),
    ("Scott", "Finance", 3300),
    ("Jen", "Finance", 3900),
    ("Jeff", "Marketing", 3000),
    ("Kumar", "Marketing", 2000),
    ("Saif", "Sales", 4100)
  )
  val df = simpleData.toDF("employee_name", "department", "salary")
  df.show()

  //Distinct all columns
  val distinctDF = df.distinct()
  println("Distinct count: "+distinctDF.count())
  distinctDF.show(false)

  val df2 = df.dropDuplicates()
  println("Distinct count: "+df2.count())
  df2.show(false)

  //Distinct using dropDuplicates
  val dropDisDF = df.dropDuplicates("department","salary")
  println("Distinct count of department & salary : "+dropDisDF.count())
  dropDisDF.show(false)

}