Easiest way to achieve this is following:
- Create your join in Tableau
- Select Data > Convert to Custom SQL from the menu
- Remove all the columns that you don't need, close the dialog
- Now to to sheet, right click on the datsource and select "Add to Saved Data Sources"
Now each time you start Tableau you'll have an easy shortcut to this datasource.
May I ask though, why Custom SQL? Tableau won't be able to perform certain optimizations since it won't look inside Custom SQL and will always execute it in full.
Why not keep it as a normal relation and just hide columns you are not interested in?