title: How do I convert an existing column to Foreign Key?
body: <p>I have a two tables.</p>
  
  <p>Table <code>a</code> looks something like this -- first row is column names, both contain characters:</p>
  
  <pre><code>id |  tractce   | someString
  1  |  "0011900" | "Label here"
  </code></pre>
  
  <p>Table <code>b</code>:</p>
  
  <pre><code>id | tractFIPS
  1  | "0011900"
  </code></pre>
  
  <p>How do I convert <code>a.tractce</code> to a foreign key column referencing <code>b.id</code>?</p>
  
  <p>So that it's:</p>
  
  <pre><code>id |  tractce | someString
  1  |  1       | "Label here"
  </code></pre>