Postgres CROSSTAB 查询无法获得所有预期的列

Question

我在 PostgreSQL 中有一个 table 是这样的 (Img1)

从这个table我正在努力实现这个(Img2)

我正在尝试使用 CROSSTAB 来执行此操作，但在这样做时我无法获得 Roll No 列。下面是我正在使用的查询。

SELECT * 
FROM CROSSTAB
('select student, subject, marks from dummy order by 1,2') 
AS FINAL
(
    Student TEXT, 
    Geography NUMERIC,
    History NUMERIC,
    Language NUMERIC,
    Maths NUMERIC,
    Music NUMERIC
);

如何实现我在 (Img2) 中显示的预期输出？

Answer 1

您可以只使用条件聚合：

select student,
       max(marks) filter (where subject = 'Music') as music,
       max(marks) filter (where subject = 'Maths') as maths,
       max(marks) filter (where subject = 'History') as history,
       max(marks) filter (where subject = 'Language') as language,
       max(marks) filter (where subject = 'Geography') as geography,
       rollno
from t
group by student, rollno;

Answer 2

至 return “额外”列，您需要 crosstab() 的 2 参数形式功能（这通常是你想要的）：

SELECT * 
FROM  crosstab(
   'SELECT student, roll_no, subject, marks
    FROM   dummy
    ORDER  BY 1'
 , $$SELECT unnest('{Geography, History, Language, Maths, Music}'::text[])$$
   ) AS final (
      "Student"   text
    , "Roll No"   text  -- extra column(s) go here
    , "Geography" int
    , "History"   int
    , "Language"  int
    , "Maths"     int
    , "Music"     int
);

db<>fiddle here

详细解释：

PostgreSQL Crosstab Query
Pivot on Multiple Columns using Tablefunc

Postgres CROSSTAB 查询无法获得所有预期的列

Postgres CROSSTAB Query not able to get all the expected Columns

sql

postgresql

pivot

crosstab

unpivot