Right. If they needed NULLs, then imho full algebraic datatype tagged unions would be the natural extension of these. Now, obviously you can implement those i

Right. If they needed NULLs, then imho full algebraic datatype tagged unions would be the natural extension of these. Now, obviously you can implement those in SQL using one column per type and one column as your discriminator tag, but that would be excruciating, especially considering how weak SQL generally is about standard libraries and reusability outside of the very fixed concepts like Views and Functions. Then you could properly define the semantics of your "missing value" as appropriate to the data. Various SQL servers have accepted the need to support stuffing multiple data-types into a single column - MSSQL has SQL_VARIANT, Orcale has ANYDATA, SQLLITE has... well, everything in SQLLITE. So between that and the existence of NULL, the ship has already sailed. May as well make it formal and schema-defined behavior using tagged unions. As the rest of the software industry has figured out, `Maybe<T>`s are better than NULLs.