如下学生表student，学生表中有姓名、分数、课程编号，需要按照课程对学生的成绩进行排序
select * from jinbo.student;
id | name | score | course
----+-------+-------+--------
5 | elic | 70 | 1
4 | dock | 100 | 1
3 | cark | 80 | 1
2 | bob | 90 | 1
1 | alice | 60 | 1
10 | jacky | 80 | 2
9 | iris | 80 | 2
8 | hill | 60 | 1
7 | grace | 50 | 2
6 | frank | 70 | 2
6 | test | | 2
(11 rows)
1、rank over () 可以把成绩相同的两名是并列，如下course = 2 的结果rank值为：1 2 2 4 5
select name,score,course,rank() over(partition by course order by score desc) as rank
from jinbo.student;
name | score | course | rank
-------+-------+--------+------
dock | 100 | 1 | 1
bob | 90 | 1 | 2
cark | 80 | 1 | 3
elic | 70 | 1 | 4
hill | 60 | 1 | 5
alice | 60 | 1 | 5
test | | 2 | 1
iris | 80 | 2 | 2
jacky | 80 | 2 | 2
frank | 70 | 2 | 4
grace | 50 | 2 | 5
(11 rows)
2、dense_rank()和rank over()很相似，可以把学生成绩并列不间断顺序排名，如下course = 2 的结果rank值为：1 2 2 3 4
select name,dense_rank() over(partition by course order by score desc) as rank
from jinbo.student;
name | score | course | rank
-------+-------+--------+------
dock | 100 | 1 | 1
bob | 90 | 1 | 2
cark | 80 | 1 | 3
elic | 70 | 1 | 4
hill | 60 | 1 | 5
alice | 60 | 1 | 5
test | | 2 | 1
iris | 80 | 2 | 2
jacky | 80 | 2 | 2
frank | 70 | 2 | 3
grace | 50 | 2 | 4
(11 rows)
3、row_number 可以把相同成绩的连续排名，如下 course = 2 的结果rank值为：1 2 3 4 5
select name,row_number() over(partition by course order by score desc) as rank
from jinbo.student;
name | score | course | rank
-------+-------+--------+------
dock | 100 | 1 | 1
bob | 90 | 1 | 2
cark | 80 | 1 | 3
elic | 70 | 1 | 4
hill | 60 | 1 | 5
alice | 60 | 1 | 6
test | | 2 | 1
iris | 80 | 2 | 2
jacky | 80 | 2 | 3
frank | 70 | 2 | 4
grace | 50 | 2 | 5
(11 rows)
使用rank over()的时候，空值是最大的，如果排序字段为null,可能造成null字段排在最前面，影响排序结果，可以如下：
rank over(partition by course order by score desc nulls last)
4、总结
- partition by 用于结果集分组，如果没有指定，会把整个结果集作为一个分组
- rank 、dense_rank 、row_numer 都是不同方式的结果集组内排序，一般都结合over 字句出现，over 字句里 会有 partition by、order by、last、first 的任意组合，如下：
rank() over(partition by a,b order by a,order by b desc);
rank() over(partition by a order by b nulls first)
rank() over(partition by a order by b nulls last)
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