title: select the rows based on unique column of each group from the table
body: <pre>
  <b>----------------
  c_id  s_id   p_id    (customer_id, service_id, provider_id)
  ---- ---- ------ </b> 
  1,     1,     1 - not elegible to select as next entry has same p_id
  1,     2,     1
  1,     3,     3
  
  2,     1,     1
  2,     2,     2
  2,     3,     3
  
  3,     1,     3 - not elegible to select as next entry has same p_id
  3,     2,     3 - not elegible to select as next entry has same p_id
  3,     3,     3
  
  </pre>
  <p>What is the cost effective way of writing the query to produce below result from above data ?</p>
  <pre>
  <b> ----------------
   c_id  s_id  p_id 
   ---- ---- ------</b> 
    1,    2,   1
    1,    3,   3
  
    2,    1,   1
    2,    2,   2
    2,    3,   3
  
    3,    3,   3
  </pre>