I'm working on joining one of my order tables with an exchange rate table to calculate USD values. Our exchange rates are populated around 3pm each day so I'm running into issues where an order was placed on a day where the rates don't exist yet. I'd like to just use the previous available day when the date doesn't exist but the things I've tried aren't working, like an NVL around the column where I expect a null value to occur and a case statement in the where clause. How can I accomplish this in my join?