错误：PG查询中不存在列“月”

Question

我的PG查询：

SELECT "Tracks"."PageId", 
       date_trunc("month", "Tracks"."createdAt") AS month, 
       count(*) 
FROM "Tracks" 
WHERE "Tracks"."PageId" IN (29,30,31) 
GROUP BY month, "Tracks"."PageId"

和我的架构：

id, createdAt, updatedAt, PageId

为什么我收到这个错误有点困惑！

Answer 1

您可以在where或group by子句中使用别名。您需要重复表达式：

SELECT "Tracks"."PageId", 
       date_trunc('month', "Tracks"."createdAt") AS month, 
       count(*) 
FROM "Tracks" 
WHERE "Tracks"."PageId" IN (29,30,31) 
GROUP BY date_trunc('month', "Tracks"."createdAt"), "Tracks"."PageId";

请注意，date_trunc()的第一个参数是varchar值，因此您需要将其放在单引号中，而不是双引号。

如果您不想重复表达式，可以将其放入派生表中：

select "PageId", month, count(*)
from (
    SELECT "Tracks"."PageId", 
           date_trunc('month', "Tracks"."createdAt") AS month
    FROM "Tracks" 
    WHERE "Tracks"."PageId" IN (29,30,31) 
) t
group by month, "PageId";

^{无关，但是：你应该真的避免引用标识符。他们是值得的，他们更麻烦}