title: unique key on two columns commutatively
body: <p>please suggest me how to apply unique key on two columns comparatively. i.e. suppose we have two columns FK_Col1 and FK_Col2 and if we insert 2 and 6 in both columns then  we should not be able to insert again 2 and 6 or 6 and 2 in both columns.</p>
  
  <p>Plaese suggest me how to achieve this.</p>
  
  <p>Thanks in advance</p>