按日期分组,当 count() 不产生任何行时为 0 [英] Grouping by date, with 0 when count() yields no lines
问题描述
我正在使用 Postgresql 9,当没有计算任何行时,我正在努力进行计数和分组.
I'm using Postgresql 9 and I'm fighting with counting and grouping when no lines are counted.
让我们假设以下架构:
create table views {
date_event timestamp with time zone ;
event_id integer;
}
让我们想象以下内容:
2012-01-01 00:00:05 2
2012-01-01 01:00:05 5
2012-01-01 03:00:05 8
2012-01-01 03:00:15 20
我想按小时分组,并计算行数.我希望我可以检索以下内容:
I want to group by hour, and count the number of lines. I wish I could retrieve the following :
2012-01-01 00:00:00 1
2012-01-01 01:00:00 1
2012-01-01 02:00:00 0
2012-01-01 03:00:00 2
2012-01-01 04:00:00 0
2012-01-01 05:00:00 0
.
.
2012-01-07 23:00:00 0
我的意思是,对于每个时间范围槽,我计算表中日期对应的行数,否则,我返回一个计数为零的行.
I mean that for each time range slot, I count the number of lines in my table whose date correspond, otherwise, I return a line with a count at zero.
以下肯定行不通(只会对计数行 > 0 的行发出声音).
The following will definitely not work (will yeld only lines with counted lines > 0).
SELECT extract ( hour from date_event ),count(*)
FROM views
where date_event > '2012-01-01' and date_event <'2012-01-07'
GROUP BY extract ( hour from date_event );
请注意,我可能还需要按分钟、小时、天、月或年分组(当然可以进行多个查询).
Please note I might also need to group by minute, or by hour, or by day, or by month, or by year (multiple queries is possible of course).
我只能使用普通的旧 sql,而且由于我的视图表可能非常大(> 100M 记录),我尽量记住性能.
I can only use plain old sql, and since my views table can be very big (>100M records), I try to keep performance in mind.
如何实现?
谢谢!
推荐答案
鉴于表中没有日期,您需要一种方法来生成它们.您可以使用 generate_series
函数:
Given that you don't have the dates in the table, you need a way to generate them. You can use the generate_series
function:
SELECT * FROM generate_series('2012-01-01'::timestamp, '2012-01-07 23:00', '1 hour') AS ts;
这将产生如下结果:
ts
---------------------
2012-01-01 00:00:00
2012-01-01 01:00:00
2012-01-01 02:00:00
2012-01-01 03:00:00
...
2012-01-07 21:00:00
2012-01-07 22:00:00
2012-01-07 23:00:00
(168 rows)
剩下的任务是使用这样的外连接来连接两个选择:
The remaining task is to join the two selects using an outer join like this :
select extract ( day from ts ) as day, extract ( hour from ts ) as hour,coalesce(count,0) as count from
(
SELECT extract ( day from date ) as day , extract ( hour from date ) as hr ,count(*)
FROM sr
where date>'2012-01-01' and date <'2012-01-07'
GROUP BY extract ( day from date ) , extract ( hour from date )
) AS cnt
right outer join ( SELECT * FROM generate_series ( '2012-01-01'::timestamp, '2012-01-07 23:00', '1 hour') AS ts ) as dtetable on extract ( hour from ts ) = cnt.hr and extract ( day from ts ) = cnt.day
order by day,hour asc;
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