MySQL8.0窗口函数实践及小结

MySQL8.0之前,做数据排名统计等相当痛苦,因为没有像Oracle、SQL SERVER 、PostgreSQL等其他数据库那样的窗口函数。但随着MySQL8.0中新增了窗口函数之后,针对这类统计就再也不是事了,本文就以常用的排序实例介绍MySQL的窗口函数。

1、准备工作

创建表及测试数据

mysql> use testdb;
Database changed
/*  创建表 */
mysql> create  table tb_score(id int primary key auto_increment,stu_no varchar(10),course varchar(50),score decimal(4,1),key idx_stuNo_course(stu_no,course));
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+------------------+
| Tables_in_testdb |
+------------------+
| tb_score         |
+------------------+

/* 新增一批测试数据 */
mysql> insert into tb_score(stu_no,course,score)values(2020001,mysql,90),(2020001,C++,85),(2020003,English,100),(2020002,mysql,50),(2020002,C++,70),(2020002,English,99);
Query OK, 6 rows affected (0.00 sec)
Records: 6  Duplicates: 0  Warnings: 0
mysql> insert into tb_score(stu_no,course,score)values(2020003,mysql,78),(2020003,C++,81),(2020003,English,80),(2020004,mysql,80),(2020004,C++,60),(2020004,English,100);
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0
mysql> insert into tb_score(stu_no,course,score)values(2020005,mysql,98),(2020005,C++,96),(2020005,English,70),(2020006,mysql,60),(2020006,C++,90),(2020006,English,70);
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0
mysql> insert into tb_score(stu_no,course,score)values(2020007,mysql,50),(2020007,C++,66),(2020007,English,76),(2020008,mysql,90),(2020008,C++,69),(2020008,English,86);
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0
mysql> insert into tb_score(stu_no,course,score)values(2020009,mysql,70),(2020009,C++,66),(2020009,English,86),(2020010,mysql,75),(2020010,C++,76),(2020010,English,81);
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0
mysql> insert into tb_score(stu_no,course,score)values(2020011,mysql,90),(2020012,C++,85),(2020011,English,84),(2020012,English,75),(2020013,C++,96),(2020013,English,88);
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0

2、统计每门课程分数的排名

根据每门课程的分数从高到低进行排名,此时,会出现分数相同时怎么处理的问题,下面就根据不同的窗口函数来处理不同场景的需求

ROW_NUMBER

由结果可以看出,分数相同时按照学号顺序进行排名

mysql> select stu_no,course,score, row_number()over(partition by course order by score desc ) rn
    -> from tb_score;
+---------+---------+-------+----+
| stu_no  | course  | score | rn |
+---------+---------+-------+----+
| 2020005 | C++     |  96.0 |  1 |
| 2020013 | C++     |  96.0 |  2 |
| 2020006 | C++     |  90.0 |  3 |
| 2020001 | C++     |  85.0 |  4 |
| 2020012 | C++     |  85.0 |  5 |
| 2020003 | C++     |  81.0 |  6 |
| 2020010 | C++     |  76.0 |  7 |
| 2020002 | C++     |  70.0 |  8 |
| 2020008 | C++     |  69.0 |  9 |
| 2020007 | C++     |  66.0 | 10 |
| 2020009 | C++     |  66.0 | 11 |
| 2020004 | C++     |  60.0 | 12 |
| 2020003 | English | 100.0 |  1 |
| 2020004 | English | 100.0 |  2 |
| 2020002 | English |  99.0 |  3 |
| 2020013 | English |  88.0 |  4 |
| 2020008 | English |  86.0 |  5 |
| 2020009 | English |  86.0 |  6 |
| 2020011 | English |  84.0 |  7 |
| 2020010 | English |  81.0 |  8 |
| 2020003 | English |  80.0 |  9 |
| 2020007 | English |  76.0 | 10 |
| 2020012 | English |  75.0 | 11 |
| 2020005 | English |  70.0 | 12 |
| 2020006 | English |  70.0 | 13 |
| 2020005 | mysql   |  98.0 |  1 |
| 2020001 | mysql   |  90.0 |  2 |
| 2020008 | mysql   |  90.0 |  3 |
| 2020011 | mysql   |  90.0 |  4 |
| 2020004 | mysql   |  80.0 |  5 |
| 2020003 | mysql   |  78.0 |  6 |
| 2020010 | mysql   |  75.0 |  7 |
| 2020009 | mysql   |  70.0 |  8 |
| 2020006 | mysql   |  60.0 |  9 |
| 2020002 | mysql   |  50.0 | 10 |
| 2020007 | mysql   |  50.0 | 11 |
+---------+---------+-------+----+
36 rows in set (0.00 sec)

DENSE_RANK

为了让分数相同时排名也相同,则可以使用DENSE_RANK函数,结果如下:

mysql> select stu_no,course,score, DENSE_RANK()over(partition by course order by score desc ) rn 
    -> from  tb_score  ;    
+---------+---------+-------+----+
| stu_no  | course  | score | rn |
+---------+---------+-------+----+
| 2020005 | C++     |  96.0 |  1 |
| 2020013 | C++     |  96.0 |  1 |
| 2020006 | C++     |  90.0 |  2 |
| 2020001 | C++     |  85.0 |  3 |
| 2020012 | C++     |  85.0 |  3 |
| 2020003 | C++     |  81.0 |  4 |
| 2020010 | C++     |  76.0 |  5 |
| 2020002 | C++     |  70.0 |  6 |
| 2020008 | C++     |  69.0 |  7 |
| 2020007 | C++     |  66.0 |  8 |
| 2020009 | C++     |  66.0 |  8 |
| 2020004 | C++     |  60.0 |  9 |
| 2020003 | English | 100.0 |  1 |
| 2020004 | English | 100.0 |  1 |
| 2020002 | English |  99.0 |  2 |
| 2020013 | English |  88.0 |  3 |
| 2020008 | English |  86.0 |  4 |
| 2020009 | English |  86.0 |  4 |
| 2020011 | English |  84.0 |  5 |
| 2020010 | English |  81.0 |  6 |
| 2020003 | English |  80.0 |  7 |
| 2020007 | English |  76.0 |  8 |
| 2020012 | English |  75.0 |  9 |
| 2020005 | English |  70.0 | 10 |
| 2020006 | English |  70.0 | 10 |
| 2020005 | mysql   |  98.0 |  1 |
| 2020001 | mysql   |  90.0 |  2 |
| 2020008 | mysql   |  90.0 |  2 |
| 2020011 | mysql   |  90.0 |  2 |
| 2020004 | mysql   |  80.0 |  3 |
| 2020003 | mysql   |  78.0 |  4 |
| 2020010 | mysql   |  75.0 |  5 |
| 2020009 | mysql   |  70.0 |  6 |
| 2020006 | mysql   |  60.0 |  7 |
| 2020002 | mysql   |  50.0 |  8 |
| 2020007 | mysql   |  50.0 |  8 |
+---------+---------+-------+----+
36 rows in set (0.00 sec)

RANK

DENSE_RANK的结果是分数相同时排名相同了,但是下一个名次是紧接着上一个名次的,如果2个并列的第1之后,下一个我想是第3名,则可以使用RANK函数实现

mysql> select stu_no,course,score, rank()over(partition by course order by score desc ) rn 
    -> from  tb_score;
+---------+---------+-------+----+
| stu_no  | course  | score | rn |
+---------+---------+-------+----+
| 2020005 | C++     |  96.0 |  1 |
| 2020013 | C++     |  96.0 |  1 |
| 2020006 | C++     |  90.0 |  3 |
| 2020001 | C++     |  85.0 |  4 |
| 2020012 | C++     |  85.0 |  4 |
| 2020003 | C++     |  81.0 |  6 |
| 2020010 | C++     |  76.0 |  7 |
| 2020002 | C++     |  70.0 |  8 |
| 2020008 | C++     |  69.0 |  9 |
| 2020007 | C++     |  66.0 | 10 |
| 2020009 | C++     |  66.0 | 10 |
| 2020004 | C++     |  60.0 | 12 |
| 2020003 | English | 100.0 |  1 |
| 2020004 | English | 100.0 |  1 |
| 2020002 | English |  99.0 |  3 |
| 2020013 | English |  88.0 |  4 |
| 2020008 | English |  86.0 |  5 |
| 2020009 | English |  86.0 |  5 |
| 2020011 | English |  84.0 |  7 |
| 2020010 | English |  81.0 |  8 |
| 2020003 | English |  80.0 |  9 |
| 2020007 | English |  76.0 | 10 |
| 2020012 | English |  75.0 | 11 |
| 2020005 | English |  70.0 | 12 |
| 2020006 | English |  70.0 | 12 |
| 2020005 | mysql   |  98.0 |  1 |
| 2020001 | mysql   |  90.0 |  2 |
| 2020008 | mysql   |  90.0 |  2 |
| 2020011 | mysql   |  90.0 |  2 |
| 2020004 | mysql   |  80.0 |  5 |
| 2020003 | mysql   |  78.0 |  6 |
| 2020010 | mysql   |  75.0 |  7 |
| 2020009 | mysql   |  70.0 |  8 |
| 2020006 | mysql   |  60.0 |  9 |
| 2020002 | mysql   |  50.0 | 10 |
| 2020007 | mysql   |  50.0 | 10 |
+---------+---------+-------+----+
36 rows in set (0.01 sec)

这样就实现了各种排序需求。

NTILE

NTILE函数的作用是对每个分组排名后,再将对应分组分成N个小组,例如

mysql> select stu_no,course,score, rank()over(partition by course order by score desc )rn,NTILE(2)over(partition by course order by score desc ) rn_group  from  tb_score;
+---------+---------+-------+----+----------+
| stu_no  | course  | score | rn | rn_group |
+---------+---------+-------+----+----------+
| 2020005 | C++     |  96.0 |  1 |        1 |
| 2020013 | C++     |  96.0 |  1 |        1 |
| 2020006 | C++     |  90.0 |  3 |        1 |
| 2020001 | C++     |  85.0 |  4 |        1 |
| 2020012 | C++     |  85.0 |  4 |        1 |
| 2020003 | C++     |  81.0 |  6 |        1 |
| 2020010 | C++     |  76.0 |  7 |        2 |
| 2020002 | C++     |  70.0 |  8 |        2 |
| 2020008 | C++     |  69.0 |  9 |        2 |
| 2020007 | C++     |  66.0 | 10 |        2 |
| 2020009 | C++     |  66.0 | 10 |        2 |
| 2020004 | C++     |  60.0 | 12 |        2 |
| 2020003 | English | 100.0 |  1 |        1 |
| 2020004 | English | 100.0 |  1 |        1 |
| 2020002 | English |  99.0 |  3 |        1 |
| 2020013 | English |  88.0 |  4 |        1 |
| 2020008 | English |  86.0 |  5 |        1 |
| 2020009 | English |  86.0 |  5 |        1 |
| 2020011 | English |  84.0 |  7 |        1 |
| 2020010 | English |  81.0 |  8 |        2 |
| 2020003 | English |  80.0 |  9 |        2 |
| 2020007 | English |  76.0 | 10 |        2 |
| 2020012 | English |  75.0 | 11 |        2 |
| 2020005 | English |  70.0 | 12 |        2 |
| 2020006 | English |  70.0 | 12 |        2 |
| 2020005 | mysql   |  98.0 |  1 |        1 |
| 2020001 | mysql   |  90.0 |  2 |        1 |
| 2020008 | mysql   |  90.0 |  2 |        1 |
| 2020011 | mysql   |  90.0 |  2 |        1 |
| 2020004 | mysql   |  80.0 |  5 |        1 |
| 2020003 | mysql   |  78.0 |  6 |        1 |
| 2020010 | mysql   |  75.0 |  7 |        2 |
| 2020009 | mysql   |  70.0 |  8 |        2 |
| 2020006 | mysql   |  60.0 |  9 |        2 |
| 2020002 | mysql   |  50.0 | 10 |        2 |
| 2020007 | mysql   |  50.0 | 10 |        2 |
+---------+---------+-------+----+----------+
36 rows in set (0.01 sec)

3、窗口函数小结

MySQL中还有许多其他的窗口函数,本文列举一些,大家可以自行测试

 
类别 函数 说明
排序 ROW_NUMBER 为表中的每一行分配一个序号,可以指定分组(也可以不指定)及排序字段
DENSE_RANK 根据排序字段为每个分组中的每一行分配一个序号。 排名值相同时,序号相同,序号中没有间隙(1,1,2,3这种)
RANK 根据排序字段为每个分组中的每一行分配一个序号。 排名值相同时,序号相同,但序号中存在间隙(1,1,3,4这种)
NTILE 根据排序字段为每个分组中根据指定字段的排序再分成对应的组
分布 PERCENT_RANK 计算各分组或结果集中行的百分数等级
CUME_DIST 计算某个值在一组有序的数据中累计的分布
前后 LEAD 返回分组中当前行之后的第N行的值。如果不存在对应行,则返回NULL。比如N=1时,第一名对应的值是第二名的,最后一名结果是NULL
LAG 返回分组中当前行之前的第N行的值。如果不存在对应行,则返回NULL。比如N=1时,第一名对应的值是是NUL,最后一名结果是倒数第2的值
首尾中 FIRST_VALUE 返回每个分组中第一名对应的字段(或表达式)的值,例如本文中可以是第一名的分数、学号等任意字段的值
LAST_VALUE 返回每个分组中最后一名对应的字段(或表达式)的值,例如本文中可以是最后一名的分数、学号等任意字段的值
NTH_VALUE

返回每个分组中排名第N的对应字段(或表达式)的值,但小于N的行对应的值是NULL

MySQL中主要的窗口函数先总结这么多,建议还是得动手实践一番。另外,MySQL5.7及之前版本的排序方式的实现很多人已总结,也建议实操一番。

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MySQL8.0窗口函数实践及小结

MySQL8.0窗口函数实践及小结

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