Postgres交叉表查詢動態pivot

Question

有誰知道如何在 Postgres 中創建以下交叉表？

例如我有下表：

Store      Month        Sales
A          Mar-2020     100
A          Feb-2020     200
B          Mar-2020     400
B          Feb-2020     500
A          Jan-2020     400
C          Apr-2020     600

我希望查詢返回以下交叉表，列標題不應是硬編碼值，而是反映第一個表中“月”列中的值：

Store    Jan-2020    Feb-2020    Mar-2020    Apr-2020
A        400         200         100         -
B        -           500         400         -
C        -           -           -           600

這可能嗎？

Answer 1

在SUM()中使用CASE表達式試試這個，這里是db-fiddle 。

select
    store,
    sum(case when month = 'Jan-2020' then sales end) as "Jan-2020",
    sum(case when month = 'Feb-2020' then sales end) as "Feb-2020",
    sum(case when month = 'Mar-2020' then sales end) as "Mar-2020",
    sum(case when month = 'Apr-2020' then sales end) as "Apr-2020"
from myTable
group by
   store
order by
   store

Output：

+---------------------------------------------------+
|store  Jan-2020    Feb-2020    Mar-2020    Apr-2020|
+---------------------------------------------------+
| A        400         200         100         null |
| B        null        500         400         null |
| C        null        null        null        600  |
+---------------------------------------------------+

如果要在null值替換為0 ，請使用coalesce()

例如

coalesce(sum(case when month = 'Jan-2020' then sales end), 0)

Answer 2

Postgres 確實有一個crosstab function，但我認為在這種情況下使用內置過濾功能很簡單：

select store,
       sum(sales) filter (where month = 'Jan-2020') as Jan_2020,
       sum(sales) filter (where month = 'Feb-2020') as Feb_2020,
       sum(sales) filter (where month = 'Mar-2020') as Mar_2020,
       sum(sales) filter (where month = 'Apr-2020') as Apr_2020
from t
group by store
order by store;

注意：這會將NULL值放在沒有對應值的列中，而不是- 。 如果您真的想要一個連字符，則需要將值轉換為字符串——這似乎是不必要的復雜。