Hi, I'd suggest reaching out to Tableau support on this one.
Hi Baker Lee I got a similar message using the version of Superstore data that comes with Tableau Prep; 'System error: Unable to run input node. Failure evaluating to database, all inputs to union must have matching column types."
See screen shot below.
This happened to me twice now in the same situation and I'm frustrated. I've had to rebuild my flow from scratch twice now, which gives me no confidence in Tableau Prep for when I use it with real data.
It happened in the same situation. I had the flow up and working well. And then I made a change to one of the data inputs that feed into a Union. In this case I "un-removed" one column that I had "removed" in the input step, and removed a different one. They're effectively the same columns, and both were measures, in decimal data format. The only visible difference is that they had different column names.
The result is that no data displays when I click on the Union 1. That result is consistent with the error message, but there's no information in the error message to help me diagnose if I did something wrong, or which fields are not "matching column types."