title: Row number per group in mysql
body: <p>I want to produce query result base on this scenario that can create row number according to crew_id and type.</p>
  
  <pre><code>id   crew_id   amount    type
   1      4       1000      AUB
   2      4       1500      AUB
   3      5       8000      CA
   4      4       1000      CA
   5      5       1000      AUB
   6      6       3000      AUB
   7      4       2000      CA
   8      6       3500      AUB
   9      4       5000      AUB
   10     5       9000      CA
   11     5       1000      CA
  </code></pre>
  
  <p>OUTPUT must be the ff:</p>
  
  <pre><code>id    crew_id   amount   type    row_number
   1      4       1000      AUB        1    
   2      4       1500      AUB        2 
   9      4       5000      AUB        3
   4      4       1000      CA         1
   7      4       2000      CA         2
   5      5       1000      AUB        1
   3      5       8000      CA         1
   10     5       9000      CA         2
   11     5       1000      CA         3
   6      6       3000      AUB        1
   6      6       3000      AUB        2
  </code></pre>
  
  <p>I want a single select statement only in this output</p>