title: SQL Add foreign key to existing column
body: <p>If I am using the following SQL command in SQL Server 2008 to update a table with a foreign key constraint:</p>
  
  <pre><code>ALTER TABLE Employees
  ADD FOREIGN KEY (UserID)
  REFERENCES ActiveDirectories(id)
  </code></pre>
  
  <p><strong><code>UserID</code></strong> being my FK column in the <code>Employees</code> table. I'm trying to reference the <strong><code>UserID</code></strong> in my <code>ActiveDirectories</code> table. I receive this error:</p>
  
  <blockquote>
    <p>Foreign key 'UserID' references invalid column 'UserID' in referencing
    table 'Employees'.</p>
  </blockquote>