Are circular references acceptable in database?

Question

When are circular references acceptable in database?

Theoretical and practical, any help is appreciated.

Answer 1

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';