如下學(xué)生表student，學(xué)生表中有姓名、分?jǐn)?shù)、課程編號(hào)，需要按照課程對(duì)學(xué)生的成績(jī)進(jìn)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 () 可以把成績(jī)相同的兩名是并列，如下course = 2 的結(jié)果rank值為：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，可以把學(xué)生成績(jī)并列不間斷順序排名，如下course = 2 的結(jié)果rank值為：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 可以把相同成績(jī)的連續(xù)排名，如下 course = 2 的結(jié)果rank值為：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()的時(shí)候，空值是最大的，如果排序字段為null, 可能造成null字段排在最前面，影響排序結(jié)果，可以如下：
rank over(partition by course order by score desc nulls last)
4、總結(jié)
partition by 用于結(jié)果集分組，如果沒(méi)有指定，會(huì)把整個(gè)結(jié)果集作為一個(gè)分組
rank 、dense_rank 、row_numer 都是不同方式的結(jié)果集組內(nèi)排序，一般都結(jié)合over 字句出現(xiàn)，over 字句里 會(huì)有 partition by、order by、last、first 的任意組合，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)
補(bǔ)充：Oracle或者PostgreSQL的row_number over 排名語(yǔ)法
PostgreSQL 和Oracle 都提供了 row_number() over() 這樣的語(yǔ)句來(lái)進(jìn)行對(duì)應(yīng)的字段排名，很是方便。MySQL卻沒(méi)有提供這樣的語(yǔ)法。
這次我提供的表結(jié)構(gòu)如下，
Table "ytt.t1"
Column | Type | Modifiers
--------+-----------------------+-----------
i_name | character varying(10) | not null
rank | integer | not null
我模擬了20條數(shù)據(jù)來(lái)做演示。
t_girl=# select * from t1 order by i_name;
i_name | rank
---------+------
Charlie | 12
Charlie | 12
Charlie | 13
Charlie | 10
Charlie | 11
Lily | 6
Lily | 7
Lily | 7
Lily | 6
Lily | 5
Lily | 7
Lily | 4
Lucy | 1
Lucy | 2
Lucy | 2
Ytt | 14
Ytt | 15
Ytt | 14
Ytt | 14
Ytt | 15
(20 rows)
在PostgreSQL下，我們來(lái)對(duì)這樣的排名函數(shù)進(jìn)行三種不同的執(zhí)行方式1：
第一種：
完整的帶有排名字段以及排序。
t_girl=# select i_name,rank, row_number() over(partition by i_name order by rank desc) as rank_number from t1;
i_name | rank | rank_number
---------+------+-------------
Charlie | 13 | 1
Charlie | 12 | 2
Charlie | 12 | 3
Charlie | 11 | 4
Charlie | 10 | 5
Lily | 7 | 1
Lily | 7 | 2
Lily | 7 | 3
Lily | 6 | 4
Lily | 6 | 5
Lily | 5 | 6
Lily | 4 | 7
Lucy | 2 | 1
Lucy | 2 | 2
Lucy | 1 | 3
Ytt | 15 | 1
Ytt | 15 | 2
Ytt | 14 | 3
Ytt | 14 | 4
Ytt | 14 | 5
(20 rows)
第二種：
帶有完整的排名字段但是沒(méi)有排序。
t_girl=# select i_name,rank, row_number() over(partition by i_name ) as rank_number from t1;
i_name | rank | rank_number
---------+------+-------------
Charlie | 12 | 1
Charlie | 12 | 2
Charlie | 13 | 3
Charlie | 10 | 4
Charlie | 11 | 5
Lily | 6 | 1
Lily | 7 | 2
Lily | 7 | 3
Lily | 6 | 4
Lily | 5 | 5
Lily | 7 | 6
Lily | 4 | 7
Lucy | 1 | 1
Lucy | 2 | 2
Lucy | 2 | 3
Ytt | 14 | 1
Ytt | 15 | 2
Ytt | 14 | 3
Ytt | 14 | 4
Ytt | 15 | 5
(20 rows)
第三種：
沒(méi)有任何排名字段，也沒(méi)有任何排序字段。
t_girl=# select i_name,rank, row_number() over() as rank_number from t1;
i_name | rank | rank_number
---------+------+-------------
Lily | 7 | 1
Lucy | 2 | 2
Ytt | 14 | 3
Ytt | 14 | 4
Charlie | 12 | 5
Charlie | 13 | 6
Lily | 7 | 7
Lily | 4 | 8
Ytt | 14 | 9
Lily | 6 | 10
Lucy | 1 | 11
Lily | 7 | 12
Ytt | 15 | 13
Lily | 6 | 14
Charlie | 11 | 15
Charlie | 12 | 16
Lucy | 2 | 17
Charlie | 10 | 18
Lily | 5 | 19
Ytt | 15 | 20
(20 rows)
以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持腳本之家。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教。
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