title: Cannot enter data into tables with inter related foreign keys
body: <p>I have created the tables with following code, but the foreign key constraints does not allow data addition. What can I do to solve this problem?</p>
  
  <pre><code>CREATE TABLE Employee(
  Ssn VARCHAR(10) PRIMARY KEY NOT NULL,
  BDate DATE,
  FName VARCHAR(25),
  MInit VARCHAR(5),
  LName VARCHAR(25),
  Address VARCHAR(40),
  Sex VARCHAR(6),
  Salary INT,
  SupervisorSsn VARCHAR(10),
  DNumber INT
  );
  CREATE TABLE
  
  CREATE TABLE Department(
  DNumber INT PRIMARY KEY NOT NULL,
  DName VARCHAR(15),
  MgrSsn VARCHAR(10),
  MgrStartDate DATE,
  NumberofEmployees INT,
  CONSTRAINT Department_MgrSsn_FK FOREIGN KEY(MgrSsn) REFERENCES Employee(Ssn) ON DELETE SET DEFAULT ON UPDATE CASCADE
  );
  
  ALTER TABLE Employee
  ADD CONSTRAINT Employee_SupervisorSsn_FK FOREIGN KEY(SupervisorSsn) REFERENCES Employee(Ssn) ON DELETE SET DEFAULT ON UPDATE CASCADE,
  ADD CONSTRAINT Employee_DNumber_FK FOREIGN KEY(DNumber) REFERENCES Department(DNumber) ON DELETE SET DEFAULT ON UPDATE CASCADE;
  </code></pre>