title: mysql group by and count rows problem
body: <p>let's say mysql is something like this </p>
  
  <pre><code>select x,y 
  from xx 
  group by y
  </code></pre>
  
  <p>i want to know how many rows that select will get, i tried to use count but it will n't return all results since i'm using group by. </p>
  
  <p>how to do that? </p>
  
  <p>Thanks</p>