I’d like to create a hierarchical table which references itself and enforces uniqueness. For example:
create table node (
id int not null generated always as identity primary key,
name varchar(32) not null,
unique (name, fk_parent_id),
foreign key (fk_parent_id) references node (id) on delete cascade
Every node has a parent node, with the exception of root node(s). Allowing fk_parent_id to be null allows for the root node(s) to exist. By uniqueness, I mean that there should only be one child node named A for a given parent node; this is why I include the parent node and the node’s name in the unique constraint.
However, derby fails for the above create statement unless “not null” is added to the spec for fk_parent_id. It says that a unique key cannot contain a nullable field. Any ideas on accomplishing the above?