Pyspark- Subquery in a case statement

Question

I am trying to run a subquery inside a case statement in Pyspark and it is throwing an exception. I am trying to create a new flag if id in one table is present in a different table.

Is this even possible in pyspark?

temp_df=spark.sql("select *, case when key in (select distinct key from Ids) then 1 else 0 end as flag from main_table")

Here is the error:

AnalysisException: 'Predicate sub-queries can only be used in a Filter

I am using Spark 2.2.1.

Asked By: kkumar

||

Source

Answer 1

This appears to be the latest detailed documentation regarding subqueries – it relates to Spark 2.0, but I haven’t seen a major update in this area since then.

The linked notebook in that reference makes it clear that indeed predicate subqueries are currently supported only within WHERE clauses.
i.e. this would work (but of course would not yield the desired result):

spark.sql("select * from main_table where id in (select distinct id from ids_table)")

You could get the same result by using a left JOIN – that’s what IN subqueries are generally translated into (for more details on that refer to the aforementioned linked notebook).

For example:

# set up some data
l1 = [('Alice', 1), ('Bob', 2), ('Eve', 3)]
df1 = sql_sc.createDataFrame(l1, ['name', 'id'])

l2 = [(1,), (2,)]
df2 = sql_sc.createDataFrame(l2, ['id'])

df1.createOrReplaceTempView("main_table")
df2.createOrReplaceTempView("ids_table")

# use a left join
spark.sql("select * from main_table m left join ids_table d on (m.id=d.id)") 
    .withColumn('flag', func.when(func.col('d.id').isNull(), 0).otherwise(1))  
    .drop('id').collect()

# result:
[Row(name='Bob', flag=1), Row(name='Eve', flag=0), Row(name='Alice', flag=1)]

Or, using pyspark sql functions rather than sql syntax:

df2 = df2.withColumnRenamed('id', 'id_faux')
df1.join(df2, df1.id == df2.id_faux, how='left') 
     .withColumn('flag', func.when(func.col('id_faux').isNull(), 0).otherwise(1)).drop('id_faux').collect()

Answered By: etov

Pyspark- Subquery in a case statement

Question:

Answers: