Getting Difference of two dataframe on Apache Spark (Scala)

Question

I have two dataframes which are large csv files which I am reading into dataframes in Spark (Scala)

First Dataframe is something like

key| col1 | col2  |
-------------------
1  | blue | house |
2  | red  | earth | 
3  | green| earth |
4  | cyan | home  |

Second dataframe is something like

key| col1 | col2  | col3
-------------------
1  | blue | house | xyz
2  | cyan | earth | xy
3  | green| mars  | xy

I want to get differences like this for common keys & common columns (keys are like primary key) in a different dataframe

key| col1         | col2           |
------------------------------------
1  | blue         | house          |
2  | red --> cyan | earth          | 
3  | green        | home--> mars   |

Below is my approach so far:

//read the files into dataframe
val src_df = read_df(file1)
val tgt_df = read_df(file2) 

//truncate dataframe to only contain common keys
val common_src = spark.sql(
"""
    select * 
    from src_df src
    where src.key IN(
        select tgt.key
        from tgt_df tgt
"""

val tgt_common = spark.sql(
"""
    select * 
    from tgt_df tgt
    where tgt.key IN(
        select src.key
        from src_df src
"""
//merge both the dataframes
val joined_df = src_common.join(tgt_common, src_common(key) === tgt_common(key), "inner")

I was unsuccessfully trying to do something like this

joined_df
.groupby(key)
.apply(some_function(?))

I have tried looking in existing solutions posted online . But I couldn't get the desired result.

PS: Also hoping the solution would be able to scale for large data

Thanks

Hedrack · Accepted Answer · 2020-02-18 22:13:52Z

1

Try the following:

spark.sql(
"""
    select 
        s.id, 
        if(s.col1 = t.col1, s.col1, s.col1 || ' --> ' || t.col1) as col1,
        if(s.col2 = t.col2, s.col2, s.col2 || ' --> ' || t.col2) as col2
    from src_df s
    inner join tgt_df t on s.id = t.id
""").show

answered Feb 18, 2020 at 22:13

Hedrack

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