title: Difference between Select Unique and Select Distinct
body: <p>I thought these were synonomous, but I wrote the following in Microsoft SQL:</p>
  
  <pre><code>Select Unique col from 
       (select col from table1 union select col from table2) alias
  </code></pre>
  
  <p>And it failed.  Changing it to </p>
  
  <pre><code>Select Distinct col from 
       (select col from table1 union select col from table2) alias
  </code></pre>
  
  <p>fixed it.  Can someone explain?</p>