title: ORA-00001 in UPDATE statement without duplicate
body: <p>I fail to understand the logic of the unique constraint when it's based on 2 fields.</p>
  
  <p>I have the following table named <code>DESCRIPTIONS</code> including 3 columns: <code>ID_DESCRIPTION</code>, <code>NAME</code>, <code>ID_DESCRIPTION_TYPE</code></p>
  
  <p>Now <code>ID_DESCRIPTION</code> is the <code>primary key</code>, and there is a <code>unique constraint</code> <code>UK_DESCRIPTION</code> on couple (<code>ID_DESCRIPTION</code>, <code>NAME</code>).</p>
  
  <p>If I try to run the following query:</p>
  
  <pre><code>UPDATE DESCRIPTIONS SET NAME = 'USA' WHERE ID_DESCRIPTION = 9255813
  </code></pre>
  
  <p>I'm getting an <code>ORA-00001</code> exception, saying that unique constraint <code>UK_DESCRIPTION</code> is violated.</p>
  
  <p>Now this would mean that the couple (<code>9255813</code>,<code>'USA'</code>) already exists right ?
  However, I don't see how this is possible since the <code>ID_DESCRIPTION</code> is a <code>primary key</code> and therefore unique AND the results of the query </p>
  
  <pre><code>SELECT * FROM  DESCRIPTIONS  WHERE ID_DESCRIPTION = 9255813
  </code></pre>
  
  <p>only return 1 result, the one I want to update.
  What am I failing to understand here ?</p>
  
  <p><a href="https://i.stack.imgur.com/zXMDY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zXMDY.png" alt="Constraint snapshot from toad"></a></p>