Ruby Gem Squeel，如何编写自联接

Posted 2023-04-12

【中文标题】Ruby Gem Squeel，如何编写自联接

【英文标题】：Ruby Gem Squeel, how to write self join

【发布时间】：2013-04-29 20:30:41

【问题描述】：

我正在尝试使用 ruby​​ gem Squeel 编写以下查询

SELECT COUNT(*)
FROM(
  SELECT
    a.end_at AS START,
    Min(b.start_at) AS END
  FROM periods AS a
  JOIN periods AS b ON b.season_id IN (1,2,3) AND a.end_at <= b.start_at
  WHERE a.season_id IN (1,2,3)
  GROUP BY a.end_at
  HAVING a.end_at < MIN(b.start_at)
) AS gaps
WHERE
  gaps.START < '2013-05-17' AND gaps.END > '2013-05-05';

知道如何实现这一目标吗？

我可以使用以下方法获取 seasons_ids：

seasons = self.seasons
Period.whereseason_id.in(seasons.selectid)

但是自我加入条件，到目前为止我不知道如何解决这个问题。

【参考方案1】：

这类似于我尝试做的here。

我会尝试使用子查询，将 Period 加入自身，如下所示：

class Period < ActiveRecord::Base
  attr_accessible :end_at, :season_id, :start_at

  belongs_to :season

  scope :in_seasons, ->(season_ids)  joinsseason.whereid.in(season_ids) 

  def self.find_gaps(start_date, end_date)
    season_ids = ["1", "2", "3"]
    scope = select[end_at.as(`gap_start`), `min(self_join.start_at)`.as(`gap_end`)]
    scope = scope.joins"LEFT JOIN (" + Period.in_seasons(season_ids).to_sql + ") AS self_join ON self_join.end_at <= periods.start_at"
    scope = scope.where(`self_join.start_at` != nil) & (`gap_start` < start_date) & (`gap_end` > end_date)
    scope = scope.group`gap_end`.havingend_at < `min(self_join.start_at)`
  end
end

在 Rails 控制台中，Period.find_gaps('2001-01-01', '2010-01-01').to_sql 产生（格式是我自己的）：

SELECT 
  \"periods\".\"end_at\" AS gap_start, 
  min(self_join.start_at) AS gap_end 
FROM \"periods\" 
  LEFT JOIN 
    (SELECT \"periods\".* 
     FROM \"periods\" 
       INNER JOIN \"seasons\" ON \"seasons\".\"id\" = \"periods\".\"season_id\" 
     WHERE \"periods\".\"id\" IN (1, 2, 3)
    ) AS self_join ON self_join.end_at <= periods.start_at 
WHERE ((self_join.start_at IS NOT NULL AND gap_start < '2001-01-01' AND gap_end > '2010-01-01')) 
GROUP BY gap_end 
HAVING \"periods\".\"end_at\" < min(self_join.start_at)

看起来你想要达到的效果......至少是内部部分。

希望对你有帮助。