> You can express a foreign key constraint that these tables must link to an employee, but not the other way around. You can’t guarantee in the limited type system of SQL that every employee row has a corresponding employee_X table, and exactly one, not more.
True. (You can in some databases using check constraints, but it is not necessarily simple) The solution is to improve the constraint model to make it much simpler to define such constraints. For example it is trivial (in most databases) to add primary key and foreign key constraints, but more complex constraints are difficult or impossible.
I think the desire for tagged unions are really an XY problem, just like the desire for array types and other composite types. The relation type itself is powerful enough. But the desire for such features show that certain patterns are too complex and un-intuitive to apply in SQL.
True. (You can in some databases using check constraints, but it is not necessarily simple) The solution is to improve the constraint model to make it much simpler to define such constraints. For example it is trivial (in most databases) to add primary key and foreign key constraints, but more complex constraints are difficult or impossible.
I think the desire for tagged unions are really an XY problem, just like the desire for array types and other composite types. The relation type itself is powerful enough. But the desire for such features show that certain patterns are too complex and un-intuitive to apply in SQL.