如何从 postgresql 中的案例名称 select

Question

我使用 postgres v14，我创建了一个包含 5 个时间列的 postgres 视图。

每个用户 ID 在第一列都有一个日期

表示每个userid超过4行

我想获得最大的一排

我的列是 (id,created_at,newstage_created_at,newstage_updated_at,newdata_created_at) 看起来像

id	created_at	newstage_created_at	newstage_updated_at	newdata_created_at
1	2020-11-23 11:28:05	2020-11-23 11:28:05	2020-11-23 11:28:05	null
1	2020-11-23 11:28:05	2020-11-23 11:28:05	2020-11-24 08:13:48	2020-11-25 08:13:48
1	2020-11-23 11:28:05	2020-10-11 05:55:36	2021-01-11 05:55:36	2020-10-11 05:55:36
2	2022-02-09 18:41:42	2022-02-09 18:41:42	2022-02-09 18:41:42	2022-02-09 18:41:42
2	2022-02-09 18:41:42	2022-02-09 18:41:42	2022-02-09 18:41:42	null
2	2022-02-09 18:41:42	2020-10-12 09:55:31	2022-02-09 18:41:42	2022-02-09 18:41:42
2	2022-02-09 18:41:42	2020-10-12 09:55:31	2022-02-09 18:41:42	2022-02-09 18:41:42

这里我需要得到具有最大列和最大行的行

我做了这个sql

case greatest(created_at, newstage_created_at ,newstage_updated_at,newdata_created_at)
when created_at then 'created_at'
when newstage_created_at then 'newstage_created_at'
when newstage_updated_at then 'newstage_updated_at'
when newdata_created_at then 'newdata_created_at'
else null
end greatestcolumn
FROM people_count_view 
WHERE 
annonce_id='1'
and 
(id,greatestcolumn) in (select id,greatestcolumn from people_count_view)
order by id

当然greatestcolumn不是列名，这是一个错误的尝试

Answer 1

不确定我是否理解你的任务是否正确，但你可以拿走这段代码并修改（通常它看起来就像你想要得到的）。

with t as (
select id,
       max(created_at)::timestamp as max_created_at,
       max(newstage_created_at) as max_newstage_created_at,
       max(newstage_updated_at) as max_newstage_updated_at,
       max(newdata_created_at) as max_newdata_created_at
from test.people_count_view
GROUP BY id
order by id)
select
    id,
    greatest(max_created_at,
        max_newstage_created_at,
        max_newstage_updated_at,
        max_newdata_created_at) as greatestcolumn,
    case greatest(max_created_at,
        max_newstage_created_at,
        max_newstage_updated_at,
        max_newdata_created_at)
        when max_created_at then 'max_created_at'
        when max_newstage_created_at then 'max_newstage_created_at'
        when max_newstage_updated_at then 'max_newstage_updated_at'
        when max_newdata_created_at then 'max_newdata_created_at'
        end as greatestcolumn_name
    from t;

output result

关于您的代码 - 请记住不要在 WHERE 条件下使用别名 (greatestcolumn)。就我而言，我将查询包装在 CTE 中以将其用作列名。其他方式 - 在 WHERE 块中，您需要使用与此别名

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