There are many external file access methods that makes the files to look like tables in the databases. You can query on a single table and join other tables for the visualizations in tableau. These joins are known as cross join in tableau, and also this would be transparent to other applications like power BI, Qlickview etc. Now if you load that database in tableau then the latency or transparency of that loaded file may not be acceptable because if someone is updating those tables on a regular basis. But you can solve this issue by selecting ‘Live’ option not the Extract option shown in Tableau interface.
You can check tableau elearning program to learn more about tableau from basic to advance level.
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