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Last tested: Jul 31, 2020
We did the auto-convert from a UDD dashboard that has merged results to a LookML Dashboard. The UDD runs as expected. However, the LookML version is giving the error "Non-base queries in merged_queries must define at least one join_fields".
If we go to the original user-defined dashboard and click "Explore From Here" on the merged results query, we might see a "Not Merged" warning in the source query from that tile. In this case we would need to fix that warning as the queries are not having a common key (join field) specified in.
We could try to have a Join field explicitly mentioned… if it is not available, we can join on the row-number(like a table calc to get the row number)