如何获得 PostgreSQL 中两个平均值之间的差异，其中平均值在一列上，最终 table 按两列分组？

Question

我想知道 value 的两个平均值之间的区别，其中每个平均值都按条件 isCool 过滤为 True 或 False，并且最终结果按 town 和 season 分组，例如

table

| id | value | isCool | town   | season |
|----|-------|--------|--------|--------|
| 0  | 1     | True   | TownA  | spring |
| 1  | 2     | False  | TownA  | winter |
| 2  | 3     | True   | TownB  | spring |
| 3  | 4     | False  | TownA  | winter |
| 4  | 5     | False  | TownB  | spring |
| 5  | 6     | True   | TownB  | winter |

我想结束 table:

| category | difference_of_is_cool_averages |
|----------|--------------------------------|
| TownA    | 2                              | <-- ABS(1 - (2 + 4)/2)
| TownB    | 0.5                            | <-- ABS(5 - (3 + 6)/2)
| spring   | 3                              | <-- ABS(5 - (3 + 1)/2)
| winter   | 3                              | <-- ABS(6 - (4 + 2)/2)

我已经尝试过了，但不幸的是，我的 PostgreSQL 技能有限，我根本没有走多远。我试过了

SELECT
   AVG(value), town
   (SELECT id, value, town, season
   FROM table
   WHERE isCool = 'True') AS TableSummary1
GROUP BY town;

但这与我想要的相去甚远。有人可以帮忙吗？这甚至可以用 PostgreSQL 实现吗？

Answer 1

您可以逆轴旋转，然后计算每组的两个条件平均值之间的差异：

select x.category, 
    abs(avg(t.value) filter(where not t.iscool) - avg(t.value) filter(where t.iscool)) diff
from mytable t
cross join lateral (values (town), (season)) as x(category)
group by x.category

如果您希望能够按照所需结果对结果集进行排序，那么我们需要跟踪原始列：

select x.category, 
    abs(avg(t.value) filter(where not t.iscool) - avg(t.value) filter(where t.iscool)) diff
from mytable t
cross join lateral (values (town, 1), (season, 2)) as x(category, grp)
group by x.category, x.grp
order by x.grp

Demo on DB Fiddle:

category |                   diff
:------- | ---------------------:
TownB    |     0.5000000000000000
TownA    | 2.00000000000000000000
winter   |     3.0000000000000000
spring   |     3.0000000000000000

Answer 2

这里Union All会帮助你。简单地通过分组 town 然后通过分组 season 并合并它们来计算一次平均值的差异。您可以像下面这样写您的查询：

select
town "Category",
round(abs(avg(value) filter (where iscool='t') - avg(value) filter (where iscool='f')),2) "difference_of_is_cool_averages"
from town
group by town

union all

select
season,
round(abs(avg(value) filter (where iscool='t') - avg(value) filter (where iscool='f')),2)
from town
group by season

DEMO

如何获得 PostgreSQL 中两个平均值之间的差异，其中平均值在一列上，最终 table 按两列分组？

How to get difference between two average values in PostgreSQL, where the averages are on a column, and the final table grouped by two columns?

sql

postgresql

average

unpivot