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I am trying to apply multiple `groupBy` operations to CSV data that I have read in with DataFrame.
```val df = DataFrame.readCSV(
    file = ClassPathResource("test.csv").file,
    delimiter = '|',
)
    .groupBy { "customerDesignation" and "contractNr" }
    .into("connectors")```
How to go from here and introduce another `groupBy` inside the  `connectors` group?

I hope I understood correctly what you mean. You want a groupBy in each of the groups of the first groupBy? I made a small notebook demonstrating this:
<https://gist.github.com/Jolanrensen/e859f9ef7b3a354f6dfe1ffd0b28259b>
Let me know if this is what you're looking for :slightly_smiling_face: