title: How do I select a fixed number of rows for each group?
body: <p>Here is some example data in a mysql table</p>
  
  <pre><code>a   b   distance
  15  44  250
  94  31  250
  30  41  250
  6   1   250
  95  18  250
  72  84  500
  14  23  500
  55  24  500
  95  8   500
  59  25  500
  40  73  500
  65  85  500
  32  50  500
  31  39  500
  22  25  500
  37  11  750
  98  39  750
  15  57  750
  9   22  750
  14  44  750
  69  22  750
  62  50  750
  89  35  750
  67  65  750
  74  37  750
  52  36  750
  66  53  750
  82  74  1000
  79  22  1000
  98  41  1000
  </code></pre>
  
  <p>How do I query this table such that I get 2 rows per distance selected at random?</p>
  
  <p>A successful query will produce something like</p>
  
  <pre><code>   a    b   distance
      30  41  250
      95  18  250
      59  25  500
      65  85  500
      15  57  750
      89  35  750
      79  22  1000
      98  41  1000
  </code></pre>