如何在 SQL 视图中将计数和总和组表示为列？

Posted 2023-05-08

【中文标题】如何在 SQL 视图中将计数和总和组表示为列？

【英文标题】：How to represent groups of counts and sums as columns in a SQL view?

【发布时间】：2019-01-17 12:22:27

【问题描述】：

基本上我在 Postgresql 中定义了 mytable

id integer,
ownername text,
length integer,
status integer

预期结果是创建一个视图，该视图将为每个所有者名称行包含以下列

 ownername  | count_in_status1 | sum_length_status1 | count_in_status2 | sum_length_status2 | count_in_status3 | sum_length_status3 | ..... | total_count | total_sum_length 

这有点难以解释，但基本上我需要每个所有者名称的计数和总和，最后是总计数和总长度。目前实际上有5种状态

尝试了以下

create view myview as     
select ownername, status, count(*), sum(length) from mytable group by ownername, status

这会返回数据，但不是以我上面介绍的最有效的方式。如何实现？

【参考方案1】：

我想这就是你想要的：

create view myview as     
    select ownername, 
           count(*) filter (where status = 1) as cnt_status_1,
           sum(length) filter (where status = 1) as len_status_1,
           count(*) filter (where status = 2) as cnt_status_2,
           sum(length) filter (where status = 2) as len_status_2,
           . . .  -- continue for each status
           count(*) as cnt,
           sum(length) as length
    from mytable
    group by ownername;

【讨论】：

你不需要状态栏。

@a_horse_with_no_name 。 . .谢谢。

【参考方案2】：

使用Filter 是一个优雅的解决方案（请参阅@Gordon Linoff 回复）

兼容很多数据库，yan也可以写：

create view myview as     
select ownername,
       sum(case when status = 1 then 1      else 0 end) as cnt_status_1,
       sum(case when status = 1 then length else 0 end) as len_status_1,
       sum(case when status = 1 then 1      else 0 end) as cnt_status_2,
       sum(case when status = 1 then length else 0 end) as len_status_2,           
       . . .  -- continue for each status
       count(*) as cnt,
       sum(length) as length
from mytable
group by ownername;

【讨论】：

其实filter()是标准的ISO SQL