ruby-on-rails – 如何按日期计数获得此postgres组包括0天

我有这个Postgres查询来选择过去30天内的所有用户并按计数分组.

@users = User.where('created_at >= ?',last_30_day).count(:order => 'DATE(created_at) ASC',:group => ["DATE(created_at)"])

问题是这样返回

2014-01-15     6
2014-01-13     2
2014-01-09     1

而且我希望得到cero结果的日子也是如此

2014-01-15     6
2014-01-14     0
2014-01-13     2
2014-01-12     0
2014-01-11     0
2014-01-10     0
2014-01-09     1

知道怎么做吗？

解决方法

要解释@mu_is_too_short答案,你可以使用generate_series这样做：

SELECT dt.series,COUNT(u.created_at)
FROM users u
RIGHT OUTER JOIN (
    SELECT 
    GENERATE_SERIES( (Now() - INTERVAL '30 day')::date,Now()::date,'1 day')::date
    AS series
) AS dt 
on DATE_Trunc('month',u.created_at) = dt.series
GROUP BY dt.series
ORDER BY dt.series DESC

产量如下：

series   | count
------------+-------
 2014-01-08 |     0
 2014-01-07 |     0
 2014-01-06 |     0
 2014-01-05 |     0
 2014-01-04 |     0
 2014-01-03 |     0
 2014-01-02 |     0
 2014-01-01 |     1
 2013-12-31 |     0

此外,ActiveRecord语法非常难看,最好的办法是在上面的纯sql中编写它,然后检索原始结果.就像是：

sql = <<-sql
    SELECT dt.series,COUNT(u.created_at)
    FROM users u
    RIGHT OUTER JOIN (
        SELECT 
        GENERATE_SERIES( (Now() - INTERVAL '30 day')::date,'1 day')::date
        AS series
    ) AS dt 
    on DATE_Trunc('month',u.created_at) = dt.series
    GROUP BY dt.series
    ORDER BY dt.series DESC
sql

records = User.connection.select_all(sql)
records.each do |record|
  puts record['series']
end

因为<< -sql是插值的,所以你可以使用里面的变量来调整你的查询.