Mysql 如何使用Codeigniter将两个不同的查询连接到一个查询中

我试图加入两个查询，以比较使用CodeIgniter（CI）的学生两个学期的结果

我可以直接使用MySQL，但是如何使用CI呢

以下是对MySQL的查询：

select x.student, x.2017, y.2018 from (SELECT a.student_id student, sum(a.marks) 2017 FROM a a 
WHERE a.year = '2017' GROUP by student
) x
LEFT JOIN
(SELECT b.student_id student, sum(b.marks) 2018 FROM b b
WHERE b.year = '2018' GROUP by student
) y on x.student = y.student

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如何在CI中实现这一点

试试这个，希望它能起作用

    $this->db->query("select x.student, x.2017, y.2018 from (SELECT 
    a.student_id student, sum(a.marks) 2017 FROM a a 
    WHERE a.year = '2017' GROUP by student
    ) x
    LEFT JOIN
    (SELECT b.student_id student, sum(b.marks) 2018 FROM b b
    WHERE b.year = '2018' GROUP by student
    ) y on x.student = y.student");