Question

在我正在处理的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

我正在尝试选择所有未参与特定日期或日期范围之间的志愿者：

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]) }

这些范围我正在与其他范围合并以过滤志愿者。

上述范围:not_engaged_between生成以下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')

问题是志愿者到目前为止可能没有任何约定，在这种情况下，查询完全忽略了那些志愿者。即使他们没有任何约定，我怎样才能让所有志愿者归还？

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join子句。

LEFT JOIN使用另一个accounts，如果它也可能丢失。但似乎您确实要检查user_type和activation_state。

Answer 2

您需要not exists才能执行此操作。这种语法在SQL上有点沉重，也许有更好的方法来实现它。

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]) }

根据另一个表中的条件选择表中的记录

2 个答案: