Question

[信息：Ubuntu 14.02，ruby 2.2.3p110，Rails 4.2.1]
我需要一个rake任务来从DB（SQLite）中获取每小时范围的平均插入次数[如00:00到00:59]。我试图做一些像：

    namespace :db do
        task results: :environment do
            tweet = Tweet.group('date(created_at)').group('hour(created_at)').average()
            puts "#{tweet}"
        end

    end

但这让我这个例外

    ActiveRecord::StatementInvalid: SQLite3::SQLException: misuse of aggregate function count(): SELECT AVG(count(*)) AS average_count_all, date(created_at) AS date_created_at, hour(created_at) AS hour_created_at FROM "tweet" GROUP BY date(created_at), hour(created_at)

有没有办法使用ActiveRecord获得每小时的平均值？

Answer 1

您可以使用嵌套的原始SQL查询：

SELECT
  AVG(hourly_count) as average_count_all
FROM
  (
    SELECT
      count(*) as hourly_count,
      date_created_at,
      hour_created_at
    FROM
      (
        SELECT
          date(created_at) as date_created_at,
          hour(created_at) as hour_created_at
        FROM
          tweet
      ) as hours
    GROUP BY
      date_created_at,
      hour_created_at
  ) as hourly_counts

这是未经测试的，但这个想法是这样的：

解析日期和小时（在hours子查询中）
获取每小时总计数（在hourly_counts子查询中）
平均每小时计数（在外部查询中）

Answer 2

我使用此answer的查询解决了我的问题：
我编辑了表名，删除了where子句，将date(from_unixtime('date'))更改为date(created_at)并将hour(from_unixtime('date'))更改为strftime('%H', created_at)（SQLite没有hour()函数）

所以我有类似的东西：

select the_hour,avg(the_count) from(
    select date(created_at) as the_day, strftime('%H', created_at) as the_hour, count(*) as the_count
    from tweet group by the_day,the_hour
) s group by the_hour

平均每小时inverval - Rails

2 个答案: