如何计算最终总和并计算百分比

Question

下面我有一个查询，其中我有学生使用每种语言的总第一语言

CTE

WITH Lang AS
(
    SELECT  
        language, 
        studentcount,
        SUM(Studentcount) AS total
    FROM    
        (SELECT 
             l.longtext AS language,
             COUNT(distinct s.studentnr) AS Studentcount
         FROM 
             student s
         JOIN
             pupil p on p.id = s.pupilid
         JOIN
             pupillanguage pl on pl.personid = p.id
         JOIN
             language l on l.id = pl.languageid
         GROUP BY 
             l.longtext
         ORDER BY 
             Studentcount DESC
        ) t 
    GROUP BY 
        language, Studentcount
)

查询

SELECT initcap(language), Studentcount, total
FROM Lang 

UNION ALL

SELECT cast(count(language) as varchar(6)) ||' Languages', null, null
FROM Lang

现在我有一个主要问题是分配学生的总和值。我需要这个，以便计算列中学生人数/学生总数的百分比。但显然我的总价值并没有给我我需要的东西。

输出

    languages   students    total
    ------------------------------
    French        734        734
    Afrikaans      93         93
    Greek         117        117
    German         55         55
    Armenian      160        160
    Malaysian       5          5
    Danish         15         15
    American        5          5
    Swedish        24         24
    Bulgarian     1043      1043

预期输出：

languages   students    Percentage
-----------------------------------
French        734          24,46
Afrikaans      93           3,12
Greek         117           3,9
German         55           1,83
Armenian      160           5,33
Malaysian       5           0,16
Danish         15           0,5
American        5           0,16
Swedish        24           0,8
Bulgarian     1043         34,76

如何计算最终总和作为计算百分比的值

Answer 1

demo:db<>fiddle

您可以使用窗口函数 (https://www.postgresql.org/docs/current/static/tutorial-window.html)：

SELECT
    language,
    students,
    SUM(students) OVER () as total,
    students::decimal / SUM(students) OVER () * 100 as percentage 
FROM
--- <YOUR QUERY>

窗口函数SUM(students) OVER () 对学生列的所有值求和，因为OVER 子句中没有给出限制框架

::decimal 强制转换是获得浮点除法 (100 / 200 = 0.5) 而不是整数除法 (100 / 200 = 0) 所必需的。

Answer 2

你可以试试下面的

with t1 as (           
        SELECT l.longtext as language,
       count(distinct s.studentnr) as Studentcount    
            FROM student s
                join pupil p on p.id = s.pupilid    
                join pupillanguage pl on pl.personid = p.id
                join language l on l.id = pl.languageid    
            group by l.longtext
            order by Studentcount desc
        ) select language,Studentcount, sum(Studentcount) over() as total,
      (Studentcount::float/sum(Studentcount) over())*100 as percentage
      from t1