How can i use MATCH and AGAINST with db_select?
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.