MySQL8.0窗口函数实践及小结
2024-10-03 00:50:53
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('','mysql',90),('','C++',85),('','English',100),('','mysql',50),('','C++',70),('','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('','mysql',78),('','C++',81),('','English',80),('','mysql',80),('','C++',60),('','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('','mysql',98),('','C++',96),('','English',70),('','mysql',60),('','C++',90),('','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('','mysql',50),('','C++',66),('','English',76),('','mysql',90),('','C++',69),('','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('','mysql',70),('','C++',66),('','English',86),('','mysql',75),('','C++',76),('','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('','mysql',90),('','C++',85),('','English',84),('','English',75),('','C++',96),('','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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