Schema Diagram Notation

[Tobias_Bussmann]

At least PostgreSQL does enforce a Foreign Key to reference a column that is unique:

The referenced columns must be the columns of a non-deferrable unique or primary key constraint in the referenced table.

[code]# create table a(i int, u int unique);
CREATE TABLE

create table b(i int references a(i));

ERROR: there is no unique constraint matching given keys for referenced table “a”

create table b(i int references a(u));

CREATE TABLE
[/code]