title: Link two tables that have transposed rows and columns
body: <p>Let's say there is a table of products:</p>
  
  <pre><code>product    | description &lt;&lt;COLUMN NAMES
  productA   | dec1
  productB   | dec2
  productC   | dec3 
  </code></pre>
  
  <p>and a table of customer data:</p>
  
  <pre><code>Customer    | zipcode &lt;&lt;COLUMN NAMES
  customer1   | 12345
  customer2   | 23456
  customer3   | 34567 
  </code></pre>
  
  <p>and a table of product ratings by customer:</p>
  
  <pre><code>product    | customer1 | customer2 | customer3 &lt;&lt;COLUMN NAMES
  productA   |  1        | 3         | 5  
  productB   |  2        | 2         | 4
  productC   |  3        | 4         | 3
  </code></pre>
  
  <p>I can link (foreign key) the table of products to the table of product ratings by customer by "product". </p>
  
  <p>How do I link (foreign key) the table of customer data to the table of product ratings by customer by "customer"?</p>