title: SQL add constraint
body: <p>I have an table employees , and I want to add an constraint which makes me able to have <strong>ONLY</strong> one employee with id_manager <strong>null</strong>.. 
  I have a sample from my code , but I think it's wrong..</p>
  
  <pre><code>ALTER TABLE employees ADD CONSTRAINT u_t UNIQUE (id_manager);
  </code></pre>
  
  <p>And this is the error I get.</p>
  
  <pre><code>SQL Error: ORA-02299: (OPREAC_IDD.U_T)
  02299. 00000 - "cannot validate (%s.%s) - duplicate keys found"
  *Cause:    an alter table validating constraint failed because the table has
         duplicate key values.
  </code></pre>