mysql實現開窗函數、Mysql實現分析函數

關鍵字：mysql實現開窗函數、Mysql實現分析函數、利用變量實現窗口函數

適用范圍：mysql5.7及以下版本，mysql8.0+ 可以直接使用窗口函數

注意，變量是從左到右順序執行的

-- 測試數據
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 )

變量會最后再計算，所以是先排序好之后，才會開始計算@num

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

 

【2】實現分組排名效果（row_number() over(partition by order by )）

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

 

--子查詢方式【取分組中前N行(排名前幾名)】

 

mysql 相關子查詢參考

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
--沒有唯一標識的表，可以用checksum來標識每行(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使用子查詢實現

create table test1_1(id int auto_increment primary key,`subject` char(20),score int);
insert into test1_1 values(null,'語文',99),(null,'語文',98),(null,'語文',97);
insert into test1_1 values(null,'數學',89),(null,'數學',88),(null,'數學',87);
insert into test1_1 values(null,'英語',79),(null,'英語',78),(null,'英語',77);

-- 根據成績，求出每個科目的前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;

-- 查詢結果【左邊】原表內容　　【右邊】需求結果，根據成績，求出每個科目的前2名

 

 

 

 

 

  

【3】實現dense_rank() over(order by)

--變量會最后再計算，所以是先排序好之后，才會開始計算@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，但該方法在mysql5.5中，只支持非0數字排序生成，字符會有大問題（任意字符被case when 'a' then else end，都會走else）

--且，賦值語句等於0時也為假

--錯誤的方式
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
　　

　　

 

--正確的方式
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

　　

 

 

--關於數字的case when 驗證

 

　　

 

關於字符、字符串 的case when驗證
　　

　　

 

【4】rank() over()

　　

 

 

　　

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測試數據代碼大部分引用自：https://blog.csdn.net/mingqing6364/article/details/82621840