带有条件的Postgre SQL数据透视表

Question

例如，我有一张桌子

date        id    event
1999-01-01  1     buy
1999-01-05  2     return
1999-01-10  3     buy
1999-02-01  1     return
1999-02-01  4     buy
1999-02-01  5     return

我需要创建一个看起来像这样的表：

month     count_buy   count_return_from_last_month
1999-01   2           ? (or none, we don't have data on this, no need to specify)
1999-02   1           1

第三列计算上个月购买的ID并返回当月的ID

我想这可以通过交叉表解决，但我找不到找到第三列条件的方法

谢谢！

Answer 1

这应该做：

WITH counts AS (
    SELECT
        to_char(date, 'YYYY-MM') AS month,
        count(*) filter (WHERE event = 'buy') AS count_buy,
        count(*) filter (WHERE event = 'return') AS count_return
    FROM data
    GROUP BY 1
)
SELECT
    month,
    count_buy,
    lag(count_return) OVER (ORDER BY month) AS count_return_from_last_month
FROM counts
ORDER BY month;

尽管请注意，要使lag始终为上个月，您需要对每个月进行计数。如果数据中有漏洞，则滞后将选择上一个可用月份。