How to get other columns when using Spark DataFrame groupby?

Question

when I use DataFrame groupby like this:

df.groupBy(df("age")).agg(Map("id"->"count")) 

I will only get a DataFrame with columns "age" and "count(id)",but in df,there are many other columns like "name".

In all,I want to get the result as in MySQL,

"select name,age,count(id) from df group by age"

What should I do when use groupby in Spark?

Answer 1

Long story short in general you have to join aggregated results with the original table. Spark SQL follows the same pre-SQL:1999 convention as most of the major databases (PostgreSQL, Oracle, MS SQL Server) which doesn't allow additional columns in aggregation queries.

Since for aggregations like count results are not well defined and behavior tends to vary in systems which supports this type of queries you can just include additional columns using arbitrary aggregate like first or last.

In some cases you can replace agg using select with window functions and subsequent where but depending on the context it can be quite expensive.