create function getNthHighestScore(N int) return int
begin
  declare M int default N-1;
  return (
    select distinct score from student
    order by score desc limit M, 1;
  )
end;
select getNthHighestScore(2);

select 
class.id class_id,
class.name class_name,
s.name student_name,
score,
rank
from (
  select *,
(
    select count(distinct score) from student s2
where s2.score >= s1.score
and s2.class_id = s1.class_id
  ) as rank 
from student s1
) as s
left join class on s.class_id = class.id
where rank <= 2;
--如果不想在from中包含select子句，也可以像如下检索，不过不显示排名
select 
class.id class_id,
class.name class_name,
s1.name name,
score
from student s1
left join class on s1.class_id = class.id
where (
select count(*) from student s2
where s2.class_id = s1.class_id
and s1.score <= s2.score) <= 2
order by s1.class_id, score desc;

update 
student,
score_correct
set student.score = score_correct.score
where student.id = score_correct.uid;

select group_concat(name) from student;

insert into student(id, name) values (), (), ()

select id, name, if(score < 60, 0, score) score from student;

select name, sex, count(*) times from student
group by name, sex
having times > 1;

"select * from (" + table + ");"

"select * from (student); drop table student; --);"