Puoi usare select_sum()
funzione di code-accenditore. Prova il seguente codice-
$query = $this->db->select_sum("tb1.amount")
->from("table1 as tb1")
->join("table2 as tb2","tb1.id = tb2.id")
->where("tb2.status",1)
->get();
$query = $query->result();
O
$query = $this->db->query('SELECT sum(tb1.amount) FROM table1 as tb1 join table2 as tb2 on tb1.id = tb2.id where tb1.status = 1');
$query = $query->result_array();