title: auto incremented number generation for a non primary key column
body: <p>I have used the following id generation strategy for primary keys. </p>
  
  <pre><code>    @Id
          @GeneratedValue(strategy = IDENTITY)
          @Column(name = "id", unique = true, nullable = false, insertable = false, updatable = false)
          private Integer id;
  </code></pre>
  
  <p>I would like to do the same for a non primary key column.
          [a] What would be the semantics of defining an automatic generation scheme for such a key
          [b] Is there a guarantee that there will not be a gap in numbers for the generated number.</p>