我建议你去CodeIgniter论坛看看这类问题。

$subQuery = $this->db->select()
->....
->get_compiled_select();

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

来源：https://forum.codeigniter.com/showthread.php?tid=70943
此外，对于您提供的查询，不需要子查询：

SELECT c.name_prov as prov, b.city, a.cases, ROW_NUMBER() OVER (PARTITION BY c.name_prov ORDER BY CAST(a.cases AS INT) DESC) AS rn 
FROM tb_cases a  
JOIN tb_city b ON a.id_city = b.id_city 
JOIN tb_prov c ON a.id_prov = c.id_prov
ORDER BY prov, rn

这应该相当于您查询。