SQL：分组，按时间排序并以正确的格式显示时间

Question

我是SQL的新手，我需要按时间排序和分组。同时，我需要以正确的时间格式显示。

select test, total 
  from (
        select to_char(last_create_date, 'YYYYmm') as "test", count (seq) as "total"
          from public.newsfeed 
         group by to_char(last_create_date, 'YYYYmm')
         order by to_char(last_create_date, 'YYYYmm')) as T1

但是，每当我想获得"MON YYYY"格式的输出时，都会收到以下错误。我该如何超越？我已经阅读了stackoverflow中非常类似的问题，他们建议使用子查询或转换。但是我不知道如何将这些应用于我的案子。

 ERROR:  column "newsfeed.last_create_date" must appear in the GROUP BY
 clause or be used in an aggregate function LINE 1: select test, total
 from (select to_char(last_create_date, 'M...
                                                 ^
 ********** Error **********

 ERROR: column "newsfeed.last_create_date" must appear in the GROUP BY
 clause or be used in an aggregate function SQL state: 42803 Character:
 41

Answer 1

Postgres允许您在select中使用列别名。因此，您可以将其编写为：

select to_char(last_create_date, 'YYYYmm') as test,
       count(seq) as total
from public.newsfeed 
group by test
order by min(last_create_date);

请注意，按最小last_create_date的顺序将结果按时间顺序排列，而不是按test的字母顺序排列。

这使得更改格式变得容易：

select to_char(last_create_date, 'Mon YYYY') as test,
       count(seq) as total
from public.newsfeed 
group by test
order by min(last_create_date);

Answer 2

尝试使用'Mon YYYY'格式

select to_char(last_create_date, 'Mon YYYY') as "test", count (seq) as total
FROM public.newsfeed 
group by to_char(last_create_date, 'Mon YYYY')
order by to_char(last_create_date, 'Mon YYYY')

Answer 3

尝试如下，无需子查询

    select to_char(last_create_date, 'MON YYYY') as "test",
    count(seq) as "total"
    FROM public.newsfeed 
    group by to_char(last_create_date, 'MON YYYY')
    order by to_char(last_create_date, 'MON YYYY')

Answer 4

错误消息指出，您需要在GROUP BY列表中包括未汇总的列，如SELECT列表中所示：

select t1.test, t1.total 
  from   
  (select to_char(last_create_date, 'MON YYYY') as test, count (seq) as "total"
     from public.newsfeed 
    group by to_char(last_create_date, 'MON YYYY')
    order by test 
  ) as t1;

但是您无需使用ORDER BY的确切格式来重新构造to_char(last_create_date, 'MON YYYY')子句，您可以在其中缩写 （ test ）。

通过这种方式，您可以用count(0)，count(1)，count(*)甚至count(test)等替换count（seq）。只要您不应用诸如count(distinct test)

之类的不同字符，它们都会得出相同的计数结果。

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