# 使用row_number排序，注意排名没有并列
select 学号,课程号,成绩,
row_number()
over(PARTITION by 课程号 ORDER BY 成绩 desc) as 排名
from score ORDER BY 课程号,排名;

# 使用rank排序，注意并列排名没有连续
select 学号,课程号,成绩,
rank()
over(PARTITION by 课程号 ORDER BY 成绩 desc) as 排名
from score ORDER BY 课程号,排名;

# 使用dense_rank排序，注意并列排名是连续的
select 学号,课程号,成绩,
dense_rank()
over(PARTITION by 课程号 ORDER BY 成绩 desc) as 排名
from score ORDER BY 课程号,排名;

select * from score as a
where (select count(*) from score as b where a.课程号=b.课程号 and a.成绩<b.成绩)<2
order by a.课程号,a.成绩 desc;

select * from score as a
where (select count(*) from score as b where a.课程号=b.课程号 and a.成绩<b.成绩)<2
order by a.课程号,a.成绩 desc;

select 课程号 from score group by 课程号;

select 学号,课程号,成绩 from score
where 课程号='0001'
order by 成绩 desc
limit 2 

-- 求最大Top2
select * from score1 as a
where(
SELECT count(*) from score1 as b where a.课程号=b.课程号 and a.成绩 < b.成绩
)<2;

(select count(*) from score as b where a.课程号=b.课程号 and a.成绩<b.成绩)=1 or
(select count(*) from score as b where a.课程号=b.课程号 and a.成绩<b.成绩)=0

(select count(b.成绩) from score as b where a.课程号=b.课程号 and a.成绩<b.成绩)=1 or
(select count(b.成绩) from score as b where a.课程号=b.课程号 and a.成绩<b.成绩)=0

/*有些國家的人口是同洲份的所有其他國的3倍或以上。列出 國家名字name 和 洲份 continent。*/
select a.name,a.continent
from world as a
where
-- a.表的第一人数除以3，与同组的相同的b表人数比较，大于或等于
(
select count(*) from world as b
where a.continent=b.continent and a.population/3>=b.population
)=
-- 得到的行数是：同一分组的行数减去一
(select count(*) from world as c where a.continent=c.continent)-1