Question

我一直在尝试在我的Rails应用程序中转换mySQL查询以使用Postgres语法，并且一直在倾注Postgres文档，特别是查看条件之间的差异。查询是：

Story.select('*', 'IF (views.id is null, 0, 1) as viewed')
  .join('left join views on(
       views.story_id = stories.id and views.user_id = ?
     )', user.id)
  .group(:id)
  .order(viewed: :asc)
  .order(id: :asc)

或作为纯查询：

select stories.*,
  IF (views.id is null, 0, 1) as viewed
from stories
left join views
  on(views.story_id = stories.id and views.user_id = 1)
where 1
group by stories.id
order by viewed, stories.id

似乎postgres使用'case'而不是IF，所以我厌倦了这样的事情：

Story.select('*', '(case when viewid is null then 0, 1 end) as viewed')
  .join('left join views on(
       views.story_id = stories.id and view.user_id = ?
     )', user.id)
  .group(:id)
  .order(viewed: :asc)
  .order(id: :asc)

但是我得到了

错误的参数数量（2为0..1）

错误...

我尝试了几种变化，但没有运气。我打算在不久的将来更多地学习SQL，但同时我觉得有点迷失。

有人可以在这里指出不同语法的细节吗？谢谢！

Answer 1

基于错误：

错误的参数数量（2为0..1）

Rails（没有postgres）告诉你某个方法正在接收两个参数，但该方法只支持或不支持参数或只支持一个参数。

查看代码，发送两个参数的唯一两种方法是select和join，所以如果你尝试的话：

select('*, (case when viewid is null then 0, 1 end) as viewed')代替 select('*', '(case when viewid is null then 0, 1 end) as viewed')

和（仅用于测试，因为不良做法）

  .join("left join views on(
       views.story_id = stories.id and view.user_id = #{user.id}
     )")

而不是

  .join('left join views on(
       views.story_id = stories.id and view.user_id = ?
     )', user.id)

Answer 2

EXISTS(...)产生一个布尔值，它非常优雅地解决了NULL情况：

select s.*
  , EXISTS (select 42 from views v
            where v.story_id = s.id
            and v.user_id = 1) as viewed
from stories s
where 1=1
  ;

如果要在viewed子句中使用ORDER BY列，则必须将上述查询推送到子查询中。

Answer 3

使用带条件的单个查询最接近postgres语法转换：

   select stories.*,
        CASE WHEN views.id is null THEN 0 ELSE 1 END as viewed
      from stories
      left join views
        on(views.story_id = stories.id and views.user_id = 1)
      where true
      group by stories.id, views.id
      order by viewed, stories.id

在我的Rails应用程序中将条件MySQL查询语法转换为Postgres

3 个答案: