title: Foreign key constraint are are incompatible
body: <p>I have a problem add FOREIGN KEY</p>
  <pre><code>CREATE TABLE Persons (
      ID int,
      LastName varchar(255),
      FirstName varchar(255),
      Address varchar(255),
      City varchar(255),
      PRIMARY KEY (ID)
  );
  
  CREATE TABLE users (
      userID int NOT NULL,
      username varchar(20) NOT NULL,
      password varchar(20) NOT NULL,
      personID int NOT NULL,
      PRIMARY KEY (userID),
      FOREIGN KEY (personID) REFERENCES Persons(ID)
  );
  </code></pre>
  <p>The error show:</p>
  <blockquote>
  <p>MySQL said: Documentation</p>
  <p>#3780 - Referencing column 'personID' and referenced column 'ID' in foreign key constraint 'users_ibfk_1' are incompatible.</p>
  </blockquote>