Comment by tabtab

I've never needed such in 3 decades of systems design. I'd like to hear the details. An explicit flag or time-stamp should be used to indicate when or if a record as been updated. To be frank, heavily reliance on Null strings usually means somebody is doing something wrong or awkward in my opinion. Null strings cause 10 problems for every 1 they solve. I stand by that and will and have defended it for hours in debates. Bring it on! (Granted, most RDBMS don't offer enough tools to easily do it correctly.)