select a , b , c  from test 
   where (a = :var1 and b = :var2) 
      or (a = :var1 and c = :var3)

##</code><code>##

select a , b , c  from test 
   where  a = :var1 and ( b = :var2 or c = :var3 )

##</code><code>##

$sth = $dbh->prepare("select a from b where (c=? and d=?) or (c=? and f=?)");
$sth->execute( $cval, $dval, $cval, $fval );

##</code><code>##

$sth = $dbh->prepare("select a from b where c=? and (d=? or f=?)");
$sth->execute( $cval, $dval, $fval );