如何在 postgresql 中旋转 2 行的简单表？

Posted 2023-03-24

【中文标题】如何在 postgresql 中旋转 2 行的简单表？

【英文标题】：how to pivot simple table of 2 rows in postgresql?

【发布时间】：2021-08-19 09:47:55

【问题描述】：

预期的表格是这样的：

good_days   bad_days
6           25

但我有这张桌子：

day_type    x
bad_days    25
good_days   6

我的代码不工作：

select * 
from (select * from main_table)t
  pivot(count(x) for day_type in ('bad_days', 'good_days')  )  as pivot_table

【参考方案1】：

有多种方法可以做到这一点

使用postgresql扩展tablefunc，其中包含crosstab方法，可以接受查询结果并旋转结果

您还可以创建自定义查询（仅当您对day_type 列值知之甚少时才有效）

WITH cte (day_type, x) AS (
        VALUES ('bad_days', 25), ('good_days', 6))
SELECT sum(good_days) AS good_days,
       sum(bad_days) AS bad_days
FROM (
          (SELECT x AS good_days,
                  0 AS bad_days
           FROM cte
           WHERE day_type = 'good_days')
      UNION ALL
          (SELECT 0 AS good_days,
                  x AS bad_days
           FROM cte
           WHERE day_type = 'bad_days')) AS foo

【参考方案2】：

一个简单的方法是条件聚合：

select sum(x) filter (where data_type = 'bad_days') as bad_days,
       sum(x) filter (where data_type = 'good_days') as good_days
from t;