Question

下表列出了多项考试中的学生成绩数据。

CREATE TABLE grades
AS
  SELECT name, exams, grade_poor, grade_fair, grade_good, grade_vgood
  FROM ( VALUES
    ( 'arun'  , 8  , 1 , 4 , 2 , 1 ),
    ( 'neha'  , 10 , 3 , 2 , 1 , 4 ),
    ( 'ram'   ,  5 , 1 , 1 , 3 , 0 ),
    ( 'radha' ,  8 , 0 , 3 , 1 , 4 )
  ) AS t(name,exams,grade_poor,grade_fair,grade_good,grade_vgood);

在vgood＆gt;的意义上对等级进行排序。好＆gt;公平＆gt;差

使用这些数据找到每个学生的第50个百分位等级是否可能（或者是否有意义）？例如 - 如果我们将数据视为一系列成绩类别，则在学生姓名为arun的情况下 - 第50个百分位数为grade_fair。

Answer 1

首先你需要取消这个。我们可以这样做......

SELECT name,
  ARRAY[grade_poor, grade_fair, grade_good, grade_vgood]
FROM grades

 name  |   array   
-------+-----------
 arun  | {1,4,2,1}
 neha  | {3,2,1,4}
 ram   | {1,1,3,0}
 radha | {0,3,1,4}

然后我们需要索引成绩......我们用CROSS JOIN LATERAL来做。我们有4行，数组为4.我们想要4 * 4行。

SELECT name, grades, gs1.x, grades[gs1.x] AS gradeqty
FROM (
  SELECT name,
    ARRAY[grade_poor, grade_fair, grade_good, grade_vgood]
  FROM grades
) AS t(name, grades)
  CROSS JOIN LATERAL generate_series(1,4) AS gs1(x)
ORDER BY name, x;


 name  |  grades   | x |  gradeqty
-------+-----------+---+----------
 arun  | {1,4,2,1} | 1 |        1
 arun  | {1,4,2,1} | 2 |        4
 arun  | {1,4,2,1} | 3 |        2
 arun  | {1,4,2,1} | 4 |        1
 neha  | {3,2,1,4} | 1 |        3
 neha  | {3,2,1,4} | 2 |        2
 neha  | {3,2,1,4} | 3 |        1
 neha  | {3,2,1,4} | 4 |        4
 radha | {0,3,1,4} | 1 |        0
 radha | {0,3,1,4} | 2 |        3
 radha | {0,3,1,4} | 3 |        1
 radha | {0,3,1,4} | 4 |        4
 ram   | {1,1,3,0} | 1 |        1
 ram   | {1,1,3,0} | 2 |        1
 ram   | {1,1,3,0} | 3 |        3
 ram   | {1,1,3,0} | 4 |        0
(16 rows)

现在剩下的是，我们需要再次CROSS JOIN LATERAL再现x（我们的等级），而不是等级

SELECT name,
  gs1.x
FROM (
  SELECT name,
    ARRAY[grade_poor, grade_fair, grade_good, grade_vgood]
  FROM grades
) AS t(name, grades)
CROSS JOIN LATERAL generate_series(1,4) AS gs1(x)
CROSS JOIN LATERAL generate_series(1,grades[gs1.x]) AS gs2(x)
ORDER BY name, gs1.x;

 name  | x 
-------+---
 arun  | 1
 arun  | 2
 arun  | 2
 arun  | 2
 arun  | 2
 arun  | 3
 arun  | 3
 arun  | 4
 neha  | 1
 neha  | 1
 neha  | 1
 neha  | 2
 neha  | 2
 neha  | 3
 neha  | 4
 neha  | 4
 neha  | 4
 neha  | 4
 radha | 2
 radha | 2
 radha | 2
 radha | 3
 radha | 4
 radha | 4
 radha | 4
 radha | 4
 ram   | 1
 ram   | 2
 ram   | 3
 ram   | 3
 ram   | 3
(31 rows)

现在我们GROUP BY name然后我们使用Ordered-Set Aggregate Functions percent_disc来完成这项工作..

SELECT name, percentile_disc(0.5) WITHIN GROUP (ORDER BY gs1.x)
FROM (
  SELECT name,
    ARRAY[grade_poor, grade_fair, grade_good, grade_vgood]
  FROM grades
) AS t(name, grades)
CROSS JOIN LATERAL generate_series(1,4) AS gs1(x)
CROSS JOIN LATERAL generate_series(1,grades[gs1.x]) AS gs2(x)
GROUP BY name ORDER BY name;

 name  | percentile_disc 
-------+-----------------
 arun  |               2
 neha  |               2
 radha |               3
 ram   |               3
(4 rows)

想进一步深入了解它......

SELECT name, (ARRAY['Poor', 'Fair', 'Good', 'Very Good'])[percentile_disc(0.5) WITHIN GROUP (ORDER BY gs1.x)]
FROM (
  SELECT name,
    ARRAY[grade_poor, grade_fair, grade_good, grade_vgood]
  FROM grades
) AS t(name, grades)
CROSS JOIN LATERAL generate_series(1,4) AS gs1(x)
CROSS JOIN LATERAL generate_series(1,grades[gs1.x]) AS gs2(x)
GROUP BY name
ORDER BY name;

 name  | array 
-------+-------
 arun  | Fair
 neha  | Fair
 radha | Good
 ram   | Good
(4 rows)

如果我们为新用户提供支持，我们可以获得稍微多变的输出。

INSERT INTO grades (name,grade_poor,grade_fair,grade_good,grade_vgood)
VALUES ('Bob', 0,0,0,100);

 name  |   array   
-------+-----------
 arun  | Fair
 Bob   | Very Good
 neha  | Fair
 radha | Good
 ram   | Good
(5 rows)

Answer 2

SELECT name, exams,
       CASE WHEN 0.5 * exams <= grade_poor
                THEN 'grade_poor'
            WHEN 0.5 * exams <= grade_poor + grade_fair
                THEN 'grade_fair'
            WHEN 0.5 * exams <= grade_poor + grade_fair + grade_good
                THEN 'grade_good'
            ELSE 'grade_vgood' END AS median_grade;

这轮比赛结束，所以neha将得分＆＃34; grade_fair＆＃34;而radha将得分＆＃34; grade_good＆＃34;。如果您想要整理，请将<=更改为<。

直方图数据的百分位数

2 个答案: