Tagged unions, if I understand what they are, can be done in SQL:

    create table Foo(
      fooId primary key autoincrement, 
      type integer /* tag/discriminant */ );
    create table FooA(fooId primary key,
      /* data specific to FooA */ );
    create table FooB(fooId primary key,
      /* data specific to FooB */ );

They're not quite markup though. Markup has a string/stream with associated records. The number of these records, their kind, and their placement varies, so it can handle ordering issues or multiple similar names or missing name (e.g. missing middle name), or ignore parts of name quite naturally.