postgresql rank() over, dense_rank(), row_number() 的区别

引原文

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,score,
      course,
      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,score,
      course,
      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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