sql - 数据库查询以生成基于时间的图表

Question

我在以下（简化）结构中有一个登录表：

id | login_time
---------
1  | 2019-02-04 18:14:30.026361+00
2  | 2019-02-04 22:10:19.720065+00
3  | 2019-02-06 15:51:53.799014+00

现在我想生成这样的图表：

https://prnt.sc/mifz6y

基本上我想显示过去 48 小时内的登录信息。

我目前的查询：

SELECT count(*), date_trunc('hour', login_time) as time_trunced FROM user_logins
WHERE login_time > now() - interval '48' hour
GROUP BY time_trunced
ORDER BY time_trunced DESC

只要每小时都有条目，这就会起作用。但是，如果在某个小时内没有登录，则不会选择任何条目，如下所示：

time_trunced | count
---------------------
12:00        | 1
13:00        | 2
15:00        | 3
16:00        | 5

我需要一个连续查询，这样我就可以简单地将计数值放入一个数组中：

time_trunced | count
---------------------
12:00        | 1
13:00        | 2
14:00        | 0 <-- This is missing
15:00        | 3
16:00        | 5

基于此，我可以简单地将查询结果转换为类似的数组[1, 2, 0, 3, 5]并将其传递给我的前端。

这对postgresql有可能吗？还是我需要实现自己的逻辑？

Answer 1

我想我会这样做：

select gs.h, count(ul.login_time)
from generate_series(date_trunc('hour', now() - interval '48 hour', date_trunc('hour', now()), interval '1 hour'
                    ) gs(h) left join
     user_logins ul
     on ul.login_time >= gs.h and
        ul.login_time < gs.h + interval '1 hour'
group by gs.h
order by gs.h;

Answer 2

这几乎肯定可以整理一下，但应该给你一些想法。夹住generate_series()尖端的道具：

SELECT t.time_trunced,coalesce(l.login_count,0) as logins 
FROM
(
 -- Generate an inline view with all hours between the min & max values in user_logins table
 SELECT date_trunc('hour',a.min_time)+ interval '1h' * b.hr_offset as time_trunced
 FROM (select min(login_time) as min_time from user_logins) a
 JOIN (select generate_series(0,(select ceil((EXTRACT(EPOCH FROM max(login_time))-EXTRACT(EPOCH FROM min(login_time)))/3600) from user_logins)::int) as hr_offset) b on true
) t
LEFT JOIN 
 (
 -- OP's original query tweaked a bit
 SELECT count(*) as login_count, date_trunc('hour', login_time) as time_trunced
 FROM user_logins
 GROUP BY time_trunced
) l on t.time_trunced=l.time_trunced
order BY 1 desc;