Re: circular relationships ok?

Let's try an even simpler and even more common example: An organizational hierarchy.

CREATE TABLE Employees
 (employee_id INTEGER PRIMARY KEY,
 employee_name VARCHAR(50) NOT NULL,
 manager_id INTEGER NOT NULL
  REFERENCES Employees (employee_id),
  CHECK (employee_id =0 OR employee_id <> manager_id));

The business rule is that each employee has a manager who is also an employee.

--
David Portas