Mysql实现开窗函数、Mysql实现分析函数！

【Mysql5.7及以下版本】
- 【测试数据】
- 【1】row_number() over(order by )
- 【2】实现分组排名效果（row_number() over(partition by order by )）
- 【3】实现dense_rank() over(order by)
- 【4】rank() over()
【5】Mysql8.0及以上，直接支持窗口函数

关键字：mysql实现开窗函数、Mysql实现分析函数、利用变量实现窗口函数

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【Mysql5.7及以下版本】

适用范围：mysql5.7及以下版本，mysql8.0+ 可以直接使用窗口函数

注意，变量是从左到右顺序执行的

【测试数据】

-- 测试数据
CREATE TABLE `tem` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`str` char(1) DEFAULT NULL,
PRIMARY KEY (`id`)
) ;

INSERT INTO `test`.`tem`(`id`, `str`) VALUES (1, 'A');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (2, 'B');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (3, 'A');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (4, 'C');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (5, 'A');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (6, 'C');
INSERT INTO `test`.`tem`(`id`, `str`) VALUES (7, 'B');

【1】row_number() over(order by )

变量会最后再计算，所以是先排序好之后，才会开始计算@num

SELECT @num := @num+1 num, id, str FROM tem, (SELECT @str := '', @num := 0) t1 ORDER BY str, id;

【2】实现分组排名效果（row_number() over(partition by order by )）

--变量方式
SELECT @num := IF(@str = str, @num + 1, 1) num, id, @str := str str FROM tem, (SELECT @str := '', @num := 0) t1 ORDER BY str, id;

--子查询方式【取分组中前N行(排名前几名)】

mysql 相关子查询参考
select * from testGroup as a where a.ID in (select ID from testGroup b where a.UserID = b.UserID order by b.OrderID limit 2) --或者 select * from testGroup a where not exists (select 1 from testGroup b where a.UserID = b.UserID and a.OrderID > b.OrderID having count(1) >= 2) --或者 select * from testGroup a where (select count(1) from testGroup b where a.UserID = b.UserID and a.ID >= b.ID) <= 2 --没有唯一标识的表，可以用checksum来标识每行(MSSQL?) select * from testGroup as a where checksum(*) in (select top 2 checksum(*) from testGroup b where a.UserID = b.UserID order by b.OrderID)

mysql使用子查询实现

create table test1_1(id int auto_increment primary key,`subject` char(20),score int); insert into test1_1 values(null,'语文',99),(null,'语文',98),(null,'语文',97); insert into test1_1 values(null,'数学',89),(null,'数学',88),(null,'数学',87); insert into test1_1 values(null,'英语',79),(null,'英语',78),(null,'英语',77); -- 根据成绩，求出每个科目的前2名 select * from test1_1; select * from test1_1 t1 where (select count(1) from test1_1 t2 where t1.subject=t2.subject and t2.score>=t1.score ) <=2;

-- 查询结果【左边】原表内容　　【右边】需求结果，根据成绩，求出每个科目的前2名

【3】实现dense_rank() over(order by)

--变量会最后再计算，所以是先排序好之后，才会开始计算@num

select id,@num:=IF(@STR=STR,@num,@num+1) rn,@str:=str str from tem t1,(select @str:='',@num:=0) t2 order by str
--

---case when 形式，但该方法在mysql5.5中，只支持非0数字排序生成，字符会有大问题（任意字符被case when 'a' then else end，都会走else）

--且，赋值语句等于0时也为假

--错误的方式 select id,str, case when @str=str then @num when @str:=str then @num:=@num+1 end as 'rn' from tem t1,(select @str:='',@num:=1) t2 order by str

--正确的方式 select id,str, case when @str=str then @num when @str:=str then @num:=@num+1 else @num:=@num+1 end as 'rn' from tem t1,(select @str:='',@num:=0) t2 order by str