我有一张学生表，上面有学号、分数和科目

CREATE TABLE IF NOT EXISTS students 
(student_id INT(3),   subject ,VARCHAR(45),   score INT(3) );

插入的数据是

insert into students values(1,'math',70);
insert into students values(1,'science',71);
insert into students values(1,'history',72);
insert into students values(1,'english',73);
insert into students values(1,'kannada',74);
insert into students values(3,'math',50);
insert into students values(3,'science',51);
insert into students values(3,'history',52);
insert into students values(3,'english',53);
insert into students values(3,'kannada',54);
insert into students values(2,'math',60);
insert into students values(2,'science',61);
insert into students values(2,'history',62);
insert into students values(2,'english',63);
insert into students values(2,'kannada',64);

我在使用查询后得到所需的输出，

select student_id,score,subject
    from
        (select @prev := '', @n:=0) init
    join
        (select @n := if(subject != @prev , 1, @n+1) as n,
                 @prev := subject,
                 student_id,score,subject from students
                 order by
                 subject asc, 
                 score desc
        ) x
        where n<=2
        order by subject, score desc;

我只是不明白这是怎么回事，为什么需要加入？这是子查询吗？from子句中的语句会在每一行数据上运行吗？有人请给我解释一下。我正在学习sql。
注意：我在网上发现这个查询与此类似，我只是根据自己的要求进行了修改，这不是我的工作。