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I'm wondering if you could be dealing with Sampled data in the flow which limits the values you see, which is why everything looks to be grouped fine. But when you run the flow it brings all the data in which includes values that have not been grouped. See if this helps: Understanding and adjusting data sampling in Tableau Prep | Tableau Software
Yes I think it’s fixed the problem. Thanks!
You're welcome! Glad it helped!
While adjusting the sampling will help in most of the scenarios, the result is still not guaranteed due to the fact that Prep is only applying the grouping algorithms on sampled data. Here is a workaround that provide guaranteed results even for extremely huge data sets(millions of rows)
The trick is to generate all unique values of a field first and then unions it with sampled data(normally one row) to make sure that Tableau Prep grouping algorithm sees all values for that field.
Thank you for sharing! That's a very useful approach