How can i use MATCH and AGAINST with db_select? I have a query like:

$query = db_query("
    SELECT *, MATCH(field1,field2) AGAINST ('$s' IN BOOLEAN MODE) AS Relevance 
    FROM {my_table} 
    ORDER BY Relevance DESC, created DESC
    ");


That is working well with db_query.
How can i use same query with db_select function for drupal 7?
Adding an expression should work:

 $query = db_select('foo');
 ...
 $expression = 'MATCH(field1,field2) AGAINST (:search IN BOOLEAN MODE)';
 $query-&gt;addExpression($expression, 'Relevance', [':search' =&gt; '$s']);
 ...


See SelectQuery::addExpression for more info.