根据另一个表中的条件选择表中的记录

Question

在我正在开发的 Rails 应用程序中，我有以下设置：

class Volunteer < AR::Base
  has_many :engagements
end

class Engagement < AR::Base
  belongs_to :volunteer
end

我正在尝试选择在特定日期或日期范围之间没有参与的所有志愿者：

scope :not_engaged_on, lambda { |q| joins(:engagements).where(["engagements.date != ?", q])}

scope :not_engaged_between, lambda { |s, e| joins(:engagements).where(["engagements.date NOT BETWEEN ? AND ?", s, e]) }

我将这些范围与其他范围合并以过滤志愿者。

上述作用域:not_engaged_between 产生以下SQL：

SELECT "volunteers".* FROM "volunteers" INNER JOIN "accounts" ON "accounts"."user_id" = "volunteers"."id" AND "accounts"."user_type" = 'Volunteer' INNER JOIN "engagements" ON "engagements"."volunteer_id" = "volunteers"."id" WHERE (accounts.activation_state = 'active') AND (engagements.date NOT BETWEEN '2012-07-30' AND '2012-08-04')

问题是志愿者可能直到现在还没有任何参与，在这种情况下，查询完全忽略了这些志愿者。即使他们没有任何参与，我如何才能让所有志愿者返回？

Answer 1

@Anthony 已经提到了NOT EXISTS。另一个选项是LEFT [OUTER] JOIN + 检查是否存在 (e.volunteer_id IS NULL)：

SELECT v.*
FROM   volunteers       v
JOIN   accounts         a ON a.user_id = v.id
LEFT  JOIN engagements e ON e.volunteer_id = v.id
                        AND e.date BETWEEN '2012-07-30' AND '2012-08-04'
WHERE  a.user_type = 'Volunteer'
AND    a.activation_state = 'active'
AND   e.volunteer_id IS NULL;

在这种情况下，您必须将条件 e.date BETWEEN '2012-07-30' AND '2012-08-04' 移动到连接子句中。

如果accounts 也可能丢失，请使用另一个LEFT JOIN。但您似乎确实想检查user_type 和activation_state。

Answer 2

您需要not exists 来执行此操作。这种语法对 SQL 来说有点重，也许有更好的 arel-way 来做到这一点。

scope :not_engaged_on, lambda { |q| where(["NOT EXISTS (SELECT ID from engagements WHERE engagements.date = ? AND volunteer_id = volunteers.id)", q])}

scope :not_engaged_between, lambda { |s, e| where(["NOT EXISTS (SELECT ID from engagements WHERE engagements.date BETWEEN ? AND ? AND volunteer_id = volunteers.id)", s, e]) }