Subquery

I don't know the syntax of mssql but in mysql you should wrap subquery with parantheses and give it an alias name:

PHP Code:

$this->db->select('*')
->from("($sql) as t", false); 


so the result query will be :

SELECT * 
FROM (select * from users) as t 

Also for grouping where clauses you can use $this->db->group_start() function. For details see documentation