INNER JOIN 上的 SQL 不同，而不是 select 上的 SQL 不同

Question

我有一个带

ActiveStorage

的 Rails 应用程序。我有一个模型

Attachment

，它在所有可能的“可附加”（机构、事件等）之间共享。

为了避免同一文件的多次上传，可能会有多个具有相同校验和的 blob。

当我列出所有附件时，我想避免重复..

所以基本上，我试图在连接表上添加一个不同的列，其中列未在最终响应中列出。

我能够使用下面的 SQL 代码列出所有校验和（不同的），但实际上我并不关心这个字段。我需要所有其他的（来自附件）。

如果我将选择从

SELECT DISTINCT active_storage_blobs.checksum

更改为

SELECT DISTINCT active_storage_blobs.checksum, "attachments".*

，则无法避免重复。

SELECT DISTINCT active_storage_blobs.checksum
FROM ((SELECT "attachments".*
       FROM "attachments"
                INNER JOIN "establishments" ON "attachments"."attachable_id" = "establishments"."id"
                INNER JOIN "establishment_managers"
                           ON "establishments"."id" = "establishment_managers"."establishment_id"
       WHERE "establishment_managers"."manager_id" = 683
         AND "attachments"."attachable_type" = 'Establishment'
       ORDER BY "attachments"."id" ASC)
      UNION
      (SELECT "attachments".*
       FROM "attachments"
                INNER JOIN "events" ON "attachments"."attachable_id" = "events"."id"
                INNER JOIN "establishments" ON "events"."establishment_id" = "establishments"."id"
                INNER JOIN "establishment_managers"
                           ON "establishments"."id" = "establishment_managers"."establishment_id"
       WHERE "establishment_managers"."manager_id" = 683
         AND "attachments"."attachable_type" = 'Event'
       ORDER BY "attachments"."id" ASC)) as attachments
         INNER JOIN "active_storage_attachments" ON "active_storage_attachments"."record_type" = 'Attachment' AND
                                                    "active_storage_attachments"."name" = 'file' AND
                                                    "active_storage_attachments"."record_id" = "attachments"."id"
         INNER JOIN "active_storage_blobs"
                    ON "active_storage_blobs"."id" = "active_storage_attachments"."blob_id"
ORDER BY active_storage_blobs.checksum ASC

Answer 1

最后，答案如下：

SELECT DISTINCT ON (active_storage_blobs.checksum) "attachments".*

整个请求如下：

SELECT DISTINCT ON (active_storage_blobs.checksum) "attachments".*
FROM ((SELECT "attachments".*
       FROM "attachments"
                INNER JOIN "establishments" ON "attachments"."attachable_id" = "establishments"."id"
                INNER JOIN "establishment_managers"
                           ON "establishments"."id" = "establishment_managers"."establishment_id"
       WHERE "establishment_managers"."manager_id" = 683
         AND "attachments"."attachable_type" = 'Establishment'
       ORDER BY "attachments"."id" ASC)
      UNION
      (SELECT "attachments".*
       FROM "attachments"
                INNER JOIN "events" ON "attachments"."attachable_id" = "events"."id"
                INNER JOIN "establishments" ON "events"."establishment_id" = "establishments"."id"
                INNER JOIN "establishment_managers"
                           ON "establishments"."id" = "establishment_managers"."establishment_id"
       WHERE "establishment_managers"."manager_id" = 683
         AND "attachments"."attachable_type" = 'Event'
       ORDER BY "attachments"."id" ASC)) as attachments
         INNER JOIN "active_storage_attachments" ON "active_storage_attachments"."record_type" = 'Attachment' AND
                                                    "active_storage_attachments"."name" = 'file' AND
                                                    "active_storage_attachments"."record_id" = "attachments"."id"
         INNER JOIN "active_storage_blobs"
                    ON "active_storage_blobs"."id" = "active_storage_attachments"."blob_id"
ORDER BY active_storage_blobs.checksum ASC