问题描述
我正在使用具有以下字段的单个表(称为documents):id，parent_id和status. parent_id字段引用同一表中的id字段. status字段的类型为ENUM('submitted', 'accepted', 'rejected').
I am working with a single table (called documents) with the following fields: id, parent_id, and status. The parent_id field refers to the id field in the same table. The status field is of type ENUM('submitted', 'accepted', 'rejected').
我想选择所有没有没有子级的documents，其中status = 'accepted'.
I would like to select all documents that have no children where status = 'accepted'.
我的第一次尝试是这样的:
My first attempt looked like this:
SELECT DISTINCT `documents`.*
FROM (`documents`)
LEFT OUTER JOIN `documents` children_documents
ON `documents`.`id` = `children_documents`.`parent_id`
WHERE `children_documents`.`id` IS NULL
OR `children_documents`.`status` != 'accepted'
问题是，仍将选择带有接受和不接受子代的文档.不应选择带有任何个被接受子级的文档.
The problem with this is that a document with both accepted and unaccepted children will still be selected. No document with any accepted children should be selected.
我觉得GROUP BY可能是我的朋友，但我不知道如何使用它来获得预期的结果.
I have a feeling GROUP BY might be my friend, but I can't figure out how I would use it to get the intended result.
推荐答案
SELECT DISTINCT `documents`.*
FROM (`documents`)
LEFT OUTER JOIN `documents` children_documents
ON `documents`.`id` = `children_documents`.`parent_id`
AND `children_documents`.`status` = 'accepted'
WHERE `children_documents`.`parent_id` IS NULL
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