title: merge two columns then group by
body: <p>I have the follwoing table and data, i want to merge loc1 and loc2 in one column and remove duplicate values from loc column then group it according to group_no column.</p>
  
  <pre><code>drop table test;
  create table test (loc1 number(9), loc2 number(9), group_no number(9));
  insert into test values(2,3,1);
  insert into test values(2,9,1);
  insert into test values(4,3,1);
  insert into test values(6,8,2);
  insert into test values(11,7,2);
  insert into test values(20,15,2);
  insert into test values(15,14,2);
  insert into test values(21,31,3);
  insert into test values(31,32,3);
  </code></pre>
  
  <p>the expected results are :</p>
  
  <pre><code>loc   group_no
  2        1
  3        1
  9        1
  4        1
  6        2
  8        2 
  11       2
  20      2 
  15       2
  21       3
  31       3
  32       3
  </code></pre>
  
  <p>regards</p>