title: Confused about foreign key constraint
body: <p>I have a general question about constraint.
  What are the difference between the following examples?</p>
  
  <pre><code>CREATE TABLE Orders (
      OrderID int NOT NULL PRIMARY KEY,
      OrderNumber int NOT NULL,
      PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
  );
  
  CREATE TABLE Orders (
      OrderID int NOT NULL,
      OrderNumber int NOT NULL,
      PersonID int,
      PRIMARY KEY (OrderID),
      CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
      REFERENCES Persons(PersonID)
  );
  </code></pre>
  
  <p>Thank you!</p>