12 years ago I wrote a recursive CTE which aggregated a bunch of accounting journal entries from pretty big SAP extracts with intermediate account also needing to be calculated in a way that rolled up to company-level values, with some tagging/indexing in the process. I remember only being able to finish the query after some kind anon helped me in an IRC channel at 4am on a weeknight... to this day I'm immensely grateful to them

I made it into a neat little Django portal with configurable permissions, interactive charts for the data, filtering, etc.

It became a ~10-min async celery task running in the background from what previously used to take the company weeks to create that report in an error prone way with macros written by someone long gone / in another department. I'm still pretty proud of that app even though it never got implemented. I got promoted, moved to a different department and don't think it ever saw the light of day, but I do have the code laying around somewhere