[gfody]

honestly I don't see how it's not - do you want to be able to union an int with a float or something? there's sql_variant or you can make your own type but that's pretty gross, violates 1NF, and is the opposite of type-safety IMO

[throwaway894345]

Tagged unions as you describe are dynamic types. With a fair amount of effort, a SQL query can perhaps use the tag column to make sense of the other columns in the union at runtime, but the type system doesn't have access to that information. You can get part of the way there with constraints, but it's tedious and incomplete.