Are circular references acceptable in database?

Question

When are circular references acceptable in database?

Theoretical and practical, any help is appreciated.

Answer 1

One of the latest additions to the Oracle hierarchical query syntax - the NOCYCLE keyword - was made for expressly this purpose - to deal with circular references in the data. I don't see anything wrong with it, and have had to deal with this kind of model before. It's not too difficult, especially in Oracle which supports deferrable constraints.

Answer 2

I guess it isn't a problem if you are using a write only database. If you plan on using the RUD part of CRUD, you'll likely run into (usually avoidable) complex issues in dealing with them.

Answer 3

Circular references should be avoided like the plague. It is possible to set up two way relationships, or even relationships to yourself (if you were a table that is), but a circular dependency is just asking for trouble.

Answer 4

rarely i run across a 1:1 relationship that is necessary and imposes a circular relationship

note that the foreign-key fields in such a relationship must be nullable, otherwise you can never delete rows from the tables

Answer 5

Consider cities and states. Each city exists within a state. Each state has a capital city.

CREATE TABLE city (
  city  VARCHAR(32),
  state VARCHAR(32) NOT NULL,
  PRIMARY KEY (city),
  FOREIGN KEY (state) REFERENCES state (state)
);

CREATE TABLE state (
  state VARCHAR(32),
  capital_city VARCHAR(32),
  PRIMARY KEY (state),
  FOREIGN KEY (capital_city) REFERENCES city (city)
);

First problem - You cannot create these tables as shown, because a foreign key can't reference a column in a table that doesn't (yet) exist. The solution is to create them without the foreign keys, and then add the foreign keys afterwards.

Second problem - you cannot insert rows into either table, as each insert will require a pre-existing row in the other table. The solution is to set one of the foreign key columns to be NULL, and insert that data in two phases. e.g.

--Create state record
INSERT INTO state (state, capital_city) VALUES ('Florida', NULL);

--Create various city records
INSERT INTO city (city, state) VALUES ('Miami', 'Florida');
INSERT INTO city (city, state) VALUES ('Tallahassee', 'Florida');
INSERT INTO city (city, state) VALUES ('Orlando', 'Florida');

--Set one of the cities as the capital
UPDATE state SET capital_city = 'Tallahassee' WHERE state = 'Florida';

Answer 6

I have seen circular references done for performance reasons. It looks ugly though, and the performance might be negligible.

Example: some bulletin boards (I think phpBB does this) have a lastpostid in the category table that is a shortcut to the last post in the thread.

This creates a circle, where the last post has a FK to the category table and the category table has a FK back to the last post.

Like I said, I don't really like it, but I've seen it done.