將所有行顯示爲兩列

Question

我有這樣的視圖。

SemesterId Course  Grade 
1   course1  A 
1   course2  B 
1   course3  C 
2   course4  D 
2   course5  A 
2   course6  B 
2   course7  C 
3   course8  D 
3   course9  A 
3   course10 B 
4   course11 C 
4   course12 D 

我想要像下面使用postgre SQL得到的結果。

SemesterId1 Course1 Grade1 SemesterId2 Course2 Grade2 
1   course1 A    2 course4 D 
1   course2 B    2 course5 A 
1   course3 C    2 course6 B 
            2 course7 C 
3   course8 D    4 course11 C 
3   course9 A    4 course12 D 
3   course10 B   

我該如何得到這個結果？

Answer 1

這種任務應該在客戶端來解決。但是，在SQL中解決這個問題也是一個有趣的問題。

select 
    sid1, course1, grade1, 
    sid2, course2, grade2 
from (
    select 
     c1.semesterid sid1, c1.course course1, c1.grade grade1, c1.rk, c1.rn, 
     c2.semesterid sid2, c2.course course2, c2.grade grade2, c2.rk, c2.rn 
    from (
     select *, dense_rank() over w rk, row_number() over w rn 
     from courses 
     where semesterid % 2 = 1 
     window w as (order by semesterid) 
     ) c1 
    full join ( 
     select *, dense_rank() over w rk, row_number() over w rn 
     from courses 
     where semesterid % 2 = 0 
     window w as (order by semesterid) 
     ) c2 
    on c1.rk = c2.rk and c1.rn = c2.rn 
    order by coalesce(c1.rk, c2.rk), coalesce(c1.rn, c2.rn) 
    ) sub 

sid1 | course1 | grade1 | sid2 | course2 | grade2 
------+----------+--------+------+----------+-------- 
    1 | course1 | A  | 2 | course4 | D 
    1 | course2 | B  | 2 | course5 | A 
    1 | course3 | C  | 2 | course6 | B 
     |   |  | 2 | course7 | C 
    3 | course8 | D  |  |   | 
    3 | course9 | A  | 4 | course11 | C 
    3 | course10 | B  | 4 | course12 | D 
(7 rows) 

Answer 2

我不知道，知道爲什麼這樣做這種方式，但你可以使用：

WITH tab_row_num AS (
    SELECT row_number() over (PARTITION BY "SemesterId") rn, "SemesterId" , "Course" , "Grade" FROM table1 
), 
joint_tab AS (
select 
    t1_3."SemesterId" AS "SemesterId1", t1_3."Course" as "Course1" , t1_3."Grade" as "Grade1", 
    t2_4."SemesterId" AS "SemesterId2", t2_4."Course" as "Course2" , t2_4."Grade" as "Grade2" 
from tab_row_num t1_3 
    full outer join tab_row_num t2_4 
     ON (t1_3."SemesterId" = 1 AND t2_4."SemesterId"=2 AND t1_3.rn=t2_4.rn) 
      OR (t1_3."SemesterId" = 3 AND t2_4."SemesterId"=4 AND t1_3.rn=t2_4.rn) 
) 
SELECT 
    COALESCE("SemesterId1"::TEXT, '') , COALESCE("Course1"::TEXT, '') , COALESCE("Grade1"::TEXT, ''), 
    COALESCE("SemesterId2"::TEXT, '') , COALESCE("Course2"::TEXT, '') , COALESCE("Grade2"::TEXT, '') 
FROM (
    SELECT * 
    FROM (
     SELECT "SemesterId1" , "Course1" , "Grade1", "SemesterId2" , "Course2" , "Grade2" 
     FROM joint_tab t 
     WHERE (t."SemesterId1" = 1 OR t."SemesterId1" IS NULL) AND (t."SemesterId2" = 2 OR t."SemesterId2" IS NULL) 
     ORDER BY "SemesterId1" NULLS LAST, "Course1" ASC, "Grade1" ASC, "SemesterId2" NULLS LAST, "Course2" ASC, "Grade2" ASC 
    ) sub1 
    UNION ALL 

    SELECT * FROM (
     SELECT "SemesterId1" , "Course1" , "Grade1", "SemesterId2" , "Course2" , "Grade2" 
     FROM joint_tab t WHERE (t."SemesterId1" = 3 OR t."SemesterId1" IS NULL) AND (t."SemesterId2" = 4 OR t."SemesterId2" IS NULL) 
     ORDER BY "SemesterId1" NULLS LAST, "Course1" ASC, "Grade1" ASC, "SemesterId2" NULLS LAST, "Course2" ASC, "Grade2" ASC 
    ) sub2 
) sub