title: Make column default to NULL explicitly
body: <p><strong>How do I make a column default to NULL <em>explicitly</em>?</strong></p>
  
  <p>I would like to declare a column in Oracle SQL Developer to be <code>NULL</code> on default. I'm aware of the fact, that <code>NULL</code> will be the default value, if I do not define any default value at all. But how do I define <code>NULL</code> as default, if I would want to do it explicitly?</p>
  
  <pre><code>-- 1: Does not work.
  ALTER TABLE MY_TABLE ADD (
    MY_COLUMN TIMESTAMP(6) DEFAULT null
  );
  
  -- 2: Does not work.
  ALTER TABLE MY_TABLE ADD (
    MY_COLUMN TIMESTAMP(6) DEFAULT NULL
  );
  
  -- 3: Does not work.
  ALTER TABLE MY_TABLE ADD (
    MY_COLUMN TIMESTAMP(6) DEFAULT (null)
  );
  
  -- 4: This works.
  ALTER TABLE MY_TABLE ADD (
    MY_COLUMN TIMESTAMP(6)
  );
  </code></pre>
  
  <p>In case 1-3 the default value will be a String ("<code>NULL</code>", "<code>null</code>" or "<code>(null)</code>"), but not an actual <code>NULL</code> value. So, what am I missing here?</p>
  
  <p><strong>// Edit:</strong></p>
  
  <p>Case (a) and (b) correspond to case 1 and 2. A text value of <code>null</code> or <code>NULL</code> is displayed in SQL Developer. Case (c) corresponds to case 4, where a <em>real</em> <code>(null)</code> value is set explicitly. The screenshots were taken on a table's <em>Columns</em> tab in SQL Developer.</p>
  
  <p><a href="http://s1.postimg.org/fclraa0dp/SQL_Developer.png" rel="noreferrer">SQL Developer http://s1.postimg.org/fclraa0dp/SQL_Developer.png</a></p>