Are you talking about when you're creating a new workbook, or just updating the data in an existing one ?
If you're creating a new workbook - YES, you do have to choose the tables & joins (because Tableau knows it's a new data source, so not necessarily the same as the previous ones you've used).
The way around this ?
- Publish the data source to Tableau Server (although this gets a bit uncomfortable if you're creating calculated fields all the time)
- Create a view in the database ... this is my preferred method, because I can force all the joins that I want (so the users can't get them wrong ) and also hide the fields that won't be reported on (surrogate keys, etc..)
Thanks Chris. To clarify, I'm referring to a workbook already published Tableau Online.