With 8.2 in the "Edit Data Source" window (with the venn diagrams) you can only use AND Join Conditions on existing DB fields.
It would be nice to apply more join conditions with constants, OR clauses, etc. in the actual data source editor. I realize you can do this with custom sql, but *I think* that most DB vendors support this feature so I could see Tableau supporting it.
I also realize you can filter based on calculated fields, and can achieve this with data blending, but let's say you have this scenario...
"I want all customers along with their sales where the total is greater than $0"
FROM Customers C
LEFT JOIN Sales S ON S.CustomerID = C.ID AND S.Total > 0
You could do this by left joining Sales to Customers on the customer id and then filter out records based on the sales.total > 0, but it defeats the purpose of putting the S.Total > 0 on the join condition allowing to have all the customer records.