Outer join relationship
Sometimes you have a business requirement where there is a measure in a Sales Order Header fact table but then you need to join to a table of zero-to-many salespeople associated with the order. The business would like to see the sales measure repeated when there are multiple salespeople. But the business would like to show a blank salesperson (without losing the sales order header row) when there are zero salespeople associated with an order. Essentially this is a left outer join to a table.
Certainly you can write a custom DAX query to do this with the NATURALLEFTOUTERJOIN DAX function, however, business users would like to be able to do this with a drag and drop tool like Power BI Desktop or Excel PivotTables.
Please add some metadata to the table relationship object in SSAS Tabular which lets you mark a relationship as an outer join. Then either the engine would interpret existing queries differently (my preference) or updated query generation engines in Power BI and Excel would send new queries to SSAS which express the outer join.