背景
一直以来，MySQL 只有针对聚合函数的汇总类功能，比如MAX, AVG 等，没有从 SQL 层针对聚合类每组展开处理的功能。不过 MySQL 开放了 UDF 接口，可以用 C 来自己写UDF，这个就增加了功能行难度。
这种针对每组展开处理的功能就叫窗口函数，有的数据库叫分析函数。
在 MySQL 8.0 之前，我们想要得到这样的结果，就得用以下几种方法来实现：
1. session 变量
2. group_concat 函数组合
3. 自己写 store routines
接下来我们用经典的 学生/课程/成绩 来做窗口函数演示
准备
学生表
mysql> show create table student \G*************************** 1. row *************************** Table: studentCreate Table: CREATE TABLE student ( sid int(10) unsigned NOT NULL, sname varchar(64) DEFAULT NULL, PRIMARY KEY (sid)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec)
课程表
mysql> show create table course\G*************************** 1. row *************************** Table: courseCreate Table: CREATE TABLE `course` ( `cid` int(10) unsigned NOT NULL, `cname` varchar(64) DEFAULT NULL, PRIMARY KEY (`cid`)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec)
成绩表
mysql> show create table score\G*************************** 1. row *************************** Table: scoreCreate Table: CREATE TABLE `score` ( `sid` int(10) unsigned NOT NULL, `cid` int(10) unsigned NOT NULL, `score` tinyint(3) unsigned DEFAULT NULL, PRIMARY KEY (`sid`,`cid`)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci1 row in set (0.00 sec)
测试数据
mysql> select * from student;+-----------+--------------+| sid | sname |+-----------+--------------+| 201910001 | 张三 || 201910002 | 李四 || 201910003 | 武松 || 201910004 | 潘金莲 || 201910005 | 菠菜 || 201910006 | 杨发财 || 201910007 | 欧阳修 || 201910008 | 郭靖 || 201910009 | 黄蓉 || 201910010 | 东方不败 |+-----------+--------------+10 rows in set (0.00 sec)mysql> select * from score;;+-----------+----------+-------+| sid | cid | score |+-----------+----------+-------+| 201910001 | 20192001 | 50 || 201910001 | 20192002 | 88 || 201910001 | 20192003 | 54 || 201910001 | 20192004 | 43 || 201910001 | 20192005 | 89 || 201910002 | 20192001 | 79 || 201910002 | 20192002 | 97 || 201910002 | 20192003 | 82 || 201910002 | 20192004 | 85 || 201910002 | 20192005 | 80 || 201910003 | 20192001 | 48 || 201910003 | 20192002 | 98 || 201910003 | 20192003 | 47 || 201910003 | 20192004 | 41 || 201910003 | 20192005 | 34 || 201910004 | 20192001 | 81 || 201910004 | 20192002 | 69 || 201910004 | 20192003 | 67 || 201910004 | 20192004 | 99 || 201910004 | 20192005 | 61 || 201910005 | 20192001 | 40 || 201910005 | 20192002 | 52 || 201910005 | 20192003 | 39 || 201910005 | 20192004 | 74 || 201910005 | 20192005 | 86 || 201910006 | 20192001 | 42 || 201910006 | 20192002 | 52 || 201910006 | 20192003 | 36 || 201910006 | 20192004 | 58 || 201910006 | 20192005 | 84 || 201910007 | 20192001 | 79 || 201910007 | 20192002 | 43 || 201910007 | 20192003 | 79 || 201910007 | 20192004 | 98 || 201910007 | 20192005 | 88 || 201910008 | 20192001 | 45 || 201910008 | 20192002 | 65 || 201910008 | 20192003 | 90 || 201910008 | 20192004 | 89 || 201910008 | 20192005 | 74 || 201910009 | 20192001 | 73 || 201910009 | 20192002 | 42 || 201910009 | 20192003 | 95 || 201910009 | 20192004 | 46 || 201910009 | 20192005 | 45 || 201910010 | 20192001 | 58 || 201910010 | 20192002 | 52 || 201910010 | 20192003 | 55 || 201910010 | 20192004 | 87 || 201910010 | 20192005 | 36 |+-----------+----------+-------+50 rows in set (0.00 sec)mysql> select * from course;+----------+------------+| cid | cname |+----------+------------+| 20192001 | mysql || 20192002 | oracle || 20192003 | postgresql || 20192004 | mongodb || 20192005 | dble |+----------+------------+5 rows in set (0.00 sec)
MySQL 8.0 之前
比如我们求成绩排名前三的学生排名，我来举个用 session 变量和 group_concat 函数来分别实现的例子：
session 变量方式
每组开始赋一个初始值序号和初始分组字段。
SELECTb.cname,a.sname,c.score, c.ranking_scoreFROMstudent a,course b,(SELECTc.*,IF(@cid = c.cid,@rn := @rn + 1,@rn := 1) AS ranking_score,@cid := c.cid AS tmpcidFROM(SELECT*FROMscoreORDER BY cid,score DESC) c,(SELECT@rn := 0 rn,@cid := '') initialize_table) cWHERE a.sid = c.sidAND b.cid = c.cidAND c.ranking_score <= 3ORDER BY b.cname,c.ranking_score;+------------+-----------+-------+---------------+| cname | sname | score | ranking_score |+------------+-----------+-------+---------------+| dble | 张三 | 89 | 1 || dble | 欧阳修 | 88 | 2 || dble | 菠菜 | 86 | 3 || mongodb | 潘金莲 | 99 | 1 || mongodb | 欧阳修 | 98 | 2 || mongodb | 郭靖 | 89 | 3 || mysql | 李四 | 100 | 1 || mysql | 潘金莲 | 81 | 2 || mysql | 欧阳修 | 79 | 3 || oracle | 武松 | 98 | 1 || oracle | 李四 | 97 | 2 || oracle | 张三 | 88 | 3 || postgresql | 黄蓉 | 95 | 1 || postgresql | 郭靖 | 90 | 2 || postgresql | 李四 | 82 | 3 |+------------+-----------+-------+---------------+15 rows in set, 5 warnings (0.01 sec)
group_concat 函数方式
利用 findinset 内置函数来返回下标作为序号使用。
SELECT*FROM(SELECTb.cname,a.sname,c.score,FIND_IN_SET(c.score, d.gp) score_rankingFROMstudent a,course b,score c,(SELECTcid,GROUP_CONCAT(scoreORDER BY score DESC SEPARATOR ',') gpFROMscoreGROUP BY cidORDER BY score DESC) dWHERE a.sid = c.sidAND b.cid = c.cidAND c.cid = d.cidORDER BY d.cid,score_ranking) yttWHERE score_ranking <= 3；+------------+-----------+-------+---------------+| cname | sname | score | score_ranking |+------------+-----------+-------+---------------+| dble | 张三 | 89 | 1 || dble | 欧阳修 | 88 | 2 || dble | 菠菜 | 86 | 3 || mongodb | 潘金莲 | 99 | 1 || mongodb | 欧阳修 | 98 | 2 || mongodb | 郭靖 | 89 | 3 || mysql | 李四 | 100 | 1 || mysql | 潘金莲 | 81 | 2 || mysql | 欧阳修 | 79 | 3 || oracle | 武松 | 98 | 1 || oracle | 李四 | 97 | 2 || oracle | 张三 | 88 | 3 || postgresql | 黄蓉 | 95 | 1 || postgresql | 郭靖 | 90 | 2 || postgresql | 李四 | 82 | 3 |+------------+-----------+-------+---------------+15 rows in set (0.00 sec)
MySQL 8.0 窗口函数
MySQL 8.0 后提供了原生的窗口函数支持，语法和大多数数据库一样，比如还是之前的例子：
用 row_number() over () 直接来检索排名。
mysql>SELECT*FROM(SELECTb.cname,a.sname,c.score,row_number() over (PARTITION BY b.cnameORDER BY c.score DESC) score_rankFROMstudent AS a,course AS b,score AS cWHERE a.sid = c.sidAND b.cid = c.cid) yttWHERE score_rank <= 3;+------------+-----------+-------+------------+| cname | sname | score | score_rank |+------------+-----------+-------+------------+| dble | 张三 | 89 | 1 || dble | 欧阳修 | 88 | 2 || dble | 菠菜 | 86 | 3 || mongodb | 潘金莲 | 99 | 1 || mongodb | 欧阳修 | 98 | 2 || mongodb | 郭靖 | 89 | 3 || mysql | 李四 | 100 | 1 || mysql | 潘金莲 | 81 | 2 || mysql | 欧阳修 | 79 | 3 || oracle | 武松 | 98 | 1 || oracle | 李四 | 97 | 2 || oracle | 张三 | 88 | 3 || postgresql | 黄蓉 | 95 | 1 || postgresql | 郭靖 | 90 | 2 || postgresql | 李四 | 82 | 3 |+------------+-----------+-------+------------+15 rows in set (0.00 sec)
那我们再找出课程 MySQL 和 DBLE 里不及格的倒数前两名学生名单。
mysql>SELECT*FROM(SELECTb.cname,a.sname,c.score,row_number () over (PARTITION BY b.cidORDER BY c.score ASC) score_rankingFROMstudent AS a,course AS b,score AS cWHERE a.sid = c.sidAND b.cid = c.cidAND b.cid IN (20192005, 20192001)AND c.score < 60) yttWHERE score_ranking < 3;+-------+--------------+-------+---------------+| cname | sname | score | score_ranking |+-------+--------------+-------+---------------+| mysql | 菠菜 | 40 | 1 || mysql | 杨发财 | 42 | 2 || dble | 武松 | 34 | 1 || dble | 东方不败 | 36 | 2 |+-------+--------------+-------+---------------+4 rows in set (0.00 sec)
到此为止，我们只是演示了row_number() over() 函数的使用方法，其他的函数有兴趣的朋友可以自己体验体验，方法都差不多。
