apache spark - How to use groupBy to collect rows into a map?

Question

Context

sqlContext.sql(s"""
SELECT
school_name,
name,
age
FROM my_table
""")

Ask

Given the above table, I would like to group by school name and collect name, age into a Map[String, Int]

For example - Pseudo-code

val df = sqlContext.sql(s"""
SELECT
school_name,
age
FROM my_table
GROUP BY school_name
""")


------------------------
school_name | name  | age
------------------------
school A | "michael"| 7 
school A | "emily"  | 5
school B | "cathy"  | 10
school B | "shaun"  | 5


df.groupBy("school_name").agg(make_map)

------------------------------------
school_name | map
------------------------------------
school A    | {"michael": 7, "emily": 5}
school B    | {"cathy": 10, "shaun": 5}

Answer 1

Following will work with Spark 2.0. You can use map function available since 2.0 release to get columns as Map.

val df1 = df.groupBy(col("school_name")).agg(collect_list(map($"name",$"age")) as "map")
df1.show(false)

This will give you below output.

+-----------+------------------------------------+
|school_name|map                                 |
+-----------+------------------------------------+
|school B   |[Map(cathy -> 10), Map(shaun -> 5)] |
|school A   |[Map(michael -> 7), Map(emily -> 5)]|
+-----------+------------------------------------+

Now you can use UDF to join individual Maps into single Map like below.

import org.apache.spark.sql.functions.udf
val joinMap = udf { values: Seq[Map[String,Int]] => values.flatten.toMap }

val df2 = df1.withColumn("map", joinMap(col("map")))
df2.show(false)

This will give required output with Map[String,Int].

+-----------+-----------------------------+
|school_name|map                          |
+-----------+-----------------------------+
|school B   |Map(cathy -> 10, shaun -> 5) |
|school A   |Map(michael -> 7, emily -> 5)|
+-----------+-----------------------------+

If you want to convert a column value into JSON String then Spark 2.1.0 has introduced to_json function.

val df3 = df2.withColumn("map",to_json(struct($"map")))
df3.show(false)

The to_json function will return following output.

+-----------+-------------------------------+
|school_name|map                            |
+-----------+-------------------------------+
|school B   |{"map":{"cathy":10,"shaun":5}} |
|school A   |{"map":{"michael":7,"emily":5}}|
+-----------+-------------------------------+