The most natural of all keys

I sometimes hear, as an argument for using surrogate keys, that there is
no natural key for a table of persons. There is. It's a compound key
consisting of your mother's primary key and a sequence number telling
which of her chldren you are. In SQL:

create table person
(
      parent integer[],
      sequence int,
      name varchar(100),
      primary key (parent, sequence)
);

insert into person (parent, sequence, name) values ({}, 0, 'Lucy');
insert into person (parent, sequence, name) values ({0}, 0, 'Albert');
insert into person (parent, sequence, name) values ({0}, 1, 'Betty');
insert into person (parent, sequence, name) values ({1,0}, 0, 'Zim');

Here, Lucy is the first human ever (she has no human ancestor). Albert
and Betty are her children, and Betty has a son, Zim.

The only problem with this is that the foreign key cannot be
described in SQL. It actually has nothing to do with it being a
self-referencing key, but that the parent and
sequence columns of the referenced row should be
parts of the parent column of the referencing row. Something
like the following: foreign key (parent[1], parent[2:]) references person
(sequence, parent). Unfortunatly, only column names are permitted
in foreign key constranits, not arbitrary expressions. Implementing
the constraint as a CHECK instead is left as an excercise to
the reader.