title: find gaps in sequence by group
body: <p>I have table where is different cities and values. I want to find gaps by cities so I have table:</p>
  
  <pre><code>City    Value
  Helsinki 1
  Helsinki 2
  Helsinki 5
  Kuopio   4
  Kuopio   5
  Joensuu  1
  Joensuu  2
  Joensuu  3
  </code></pre>
  
  <p>And I want to get table where is gaps by cities so</p>
  
  <pre><code> City     Value
   Helsinki 3
   Helsinki 4
   Kuopio   1
   Kuopio   2
   Kuopio   3
   Joensuu  4
   Joensuu  5
  </code></pre>
  
  <p>I know that I can use conncet by </p>
  
  <pre><code>select  rownum from dual connect by level&lt;= 6 minus select value from table
  </code></pre>
  
  <p>but I don't know, how I can do this by group.</p>