title: Create unique constraint with nvl in Oracle
body: <p>How can I create unique constraint when I need to treat null values as equals. </p>
  
  <p>For</p>
  
  <blockquote>
    <p>alter table T1 add constraint T1_UN
    unique (nvl(C1, ' '))</p>
  </blockquote>
  
  <p>i get</p>
  
  <pre><code> ORA-00904: : invalid identifier
  </code></pre>
  
  <p>points on nvl</p>
  
  <p>Thanks!</p>