mysql实现开窗函数、Mysql实现分析函数

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

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
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

　　

--关于数字的case when 验证

　　

关于字符、字符串 的case when验证
　　

　　

【4】rank() over()

　　

　　

---------------------
测试数据代码大部分引用自：https://blog.csdn.net/mingqing6364/article/details/82621840