Tableau currently supports blending of data sources using what is effectively a left join.
The ability to create a union of data sources would also be beneficial. This would allow for reporting across distributed databases without requiring the complexity of setting up links between the databases.
As an example, you may have 4 manufacturing sites that each have instances of a reporting database. With the ability to create a union of data sources we could:
- create a data source definition for each site. This data source would contain only the information for that site and would be useful for local reporting.
- create a report that does a union across those data sources. This report would then be useful for a higher level overview of operations (i.e. comparing site-to-site performance).
Some benefits of doing this in the BI layer (as opposed to the data layer) would be:
- simpler database configuration (no need to maintain link definitions in the databases just for reporting)
- less redundant SQL (i.e. no need to create one data source with SQL for single site reports and another data source with linked query SQL for the multi site reports).
- more flexibility in managing which sites are connected to any given report (i.e. you could create a report comparing just 2 of the 4 reports without needing to change any SQL