title: SQL command that count number of rows after group by c# winforms
body: <p>I have a table called Order(Id,Number,Date).
  "Id" and "Number" are main keys.</p>
  
  <p>table Order</p>
  
  <pre><code>Id Number Date
  0   1     1995   
  0   2     1995        
  0   3     1995   
  1   1     1999   
  1   2     1999   
  2   1     2001
  </code></pre>
  
  <p>I need the number of rows but without duplicates (in this example: 3 ) 
  I know that the "group by Id" will give me: Id(0,1,2)
  but I cant count it after the group by.</p>