title: Alternative for exists query
body: <p>I have the following query :</p>
  
  <pre><code>update tab1.abc,
  tab1.sbd = (select tab2.abc, tab2.sbd from tab2,tab1 where --some conditions)
  where exists(select tab2.abc, tab2.sbd from tab2,tab1 where --some conditions)
  </code></pre>
  
  <p>Now my task is to optmize this query.   </p>
  
  <p>I believe removing the exists clause or combining both the where clauses would help a lot.
  But how to do it ?</p>
  
  <p><strong>PS</strong> : exists clause is in place because I want the number of rows updated to be zero if the select clause returns zero rows.</p>