如何用IF / case选择内连接字段？ [英] How to choose inner join field with IF/case?

问题描述

有点棘手。

我正在清理一些数据，但由于数据的大小，它需要相当长的一段时间要做，因此我需要在升级执行时运行该应用程序。所以，我有一张评论表和一张照片表。每条评论记录都存储在 object_id 中的 photo_d_id 或存储在 object_id 中的 photo_id ：

I am in the middle of cleaning up some data but because of the size of the data, it's going to take quite a while to do and thus I need the app to run while the upgrade is performing. So, I have a comments table and a photos table. Each comment record has either the photo_d_id stored in object_id or the photo_id stored in object_id:

if comment_type = 8 then object_id = photo.photo_p_id
if comment_type = 17 then object_id = photo.photo_id

我需要内部加入查询中的照片表，但需要根据照片表中的正确字段执行内部连接的 comment_type 即可。这是不可能的，但举一个我想做的例子：

I need to inner join the photo table in the query but need to perform the innerjoin on the correct field from the photo table based off the comment_type. This isn't possible, but gives an example of what I want to do:

select *.comments,*.photos 
FROM comments
CASE
   when comments.comment_type = 8 then inner join photos on comments.object_id = photo.photo_p_id
   when comments.comment_type = 17 then inner join photos on comments.object_id = photo.photo_id

任何想法？

推荐答案

尝试此查询（已修改） -

Try this query (edited) -

SELECT c.*,
  CASE c.comment_type WHEN 8 THEN p1.photo_p_id WHEN 17 THEN p2.photo_id ELSE NULL END photo_id,
  CASE c.comment_type WHEN 8 THEN p1.column1 WHEN 17 THEN p2.column1 ELSE NULL END column1
FROM comments c
  LEFT JOIN photos p1
    ON c.object_id = p1.photo_p_id
  LEFT JOIN photos p2
    ON c.object_id = p2.photo_id

另一个变种 -

SELECT c.*, p.*
FROM comments c
  JOIN photos p
    ON c.comment_type = 8 AND c.object_id = p.photo_p_id OR c.comment_type = 17 AND c.object_id = p.photo_id

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