title: Oracle Unique Constraint with Expression
body: <p>Does <code>Oracle</code> support constraints with expressions like so?</p>
  
  <p>Notice Z = 'N'</p>
  
  <pre><code>ALTER TABLE A ADD CONSTRAINT U_A_KEY UNIQUE(X,Y,Z = 'N');
  </code></pre>
  
  <p>Is this <code>Unique constraint</code> possible?</p>
  
  <p>Example:</p>
  
  <pre><code>INSERT INTO A VALUES('X','Y','N');  --OK
  INSERT INTO A VALUES('X','Y','Y');  --OK
  INSERT INTO A VALUES('X','Y','Y');  --OK
  INSERT INTO A VALUES('X','Y','N');  --VOLIATION
  </code></pre>