重组Postgrest查询

Question

我在postgresql上使用了一个返回这样结果的查询。

schedule |     day    |   subject   | grade   |
===============================================
06 - 08  |    Monday  |   Biology   | Second  |
-----------------------------------------------
08 - 10  |    Monday  |  Chemistry  | Third   |
-----------------------------------------------
06 - 08  |   Tuesday  |   Math      | Second  |
-----------------------------------------------
10 - 12  |  Wednesday |   English   | Second  |
-----------------------------------------------
10 - 12  |    Friday  |   Language  | Second  |
-----------------------------------------------

我需要一个结果或像这样重新组织这个查询。

schedule |   Monday  | Tuesday| Wednesday | Friday  | grade
==============================================================
06 - 08  |  Biology  |   Math | null      | null    | second
--------------------------------------------------------------
10 - 12  |  null     |   null | English   |Language | second
--------------------------------------------------------------

我感谢任何帮助或建议。

Thnks。

Answer 1

一种方法是条件聚合：

select schedule,
       max(case when day = 'Monday' then subject end) as Monday,
       max(case when day = 'Tuesday' then subject end) as Tuesday,
       max(case when day = 'Wednesday' then subject end) as Wednesday,
       max(case when day = 'Friday' then subject end) as Friday,
       min(grade) as grade,
from t
group by schedule;

我不确定grade的来源;以上只是猜测。