mysql - 如何过滤搜索属于已删除帖子的答案？

Question

我有一个这样的表:

// questions_and_answers
+----+---------------+------------------------+---------+---------+
| id |    subject    |           body         | related | deleted |
+----+---------------+------------------------+---------+---------+
| 1  | subject1      | question1              | NULL    | 0       |
| 2  |               | answer1                | 1       | 0       |
| 3  | subject2      | question2              | NULL    | 0       |
| 4  |               | answer2                | 3       | 1       |
| 5  |               | answer3                | 3       | 0       |
| 6  | subject3      | question3              | NULL    | 1       |
| 7  |               | answer4                | 6       | 0       |
+----+---------------+------------------------+---------+---------+
-- related column contains either NULL for questions or the id of its question for answers

如上表所示，answer2 和 question3 都被删除了。所以我想在搜索中过滤它们。重点是，我也想过滤 answer4。因为它属于删除的问题(question3)。知道如何进行此类过滤吗？

---

这是我当前的查询:

SELECT qa.*
FROM questions_and_answers AS qa
WHERE MATCH(subject,body) AGAINST (:entry)
  AND deleted = 0
ORDER BY id DESC
LIMIT :page, 10;

如您所见，它只过滤已删除的帖子。它不关心过滤属于已删除问题的答案。

Answer 1

如果只有一个级别的关系——对于任何行r，另一行r'存在，那么r' .related = r.id, r.related IS NULL -- 你可以将问题与答案左连接并检查问题的已删除，或者使用NOT EXISTS 来检查是否不存在已删除的父记录。

LEFT JOIN 变体:

SELECT qa1.*
       FROM questions_and_answers qa1
            LEFT JOIN questions_and_answers qa2
                      ON qa2.id = qa1.related
       WHERE MATCH(qa1.subject, qa1.body) AGAINST (:entry)
             AND qa1.deleted = 0
             AND coalesce(qa2.deleted, 0) = 0
       ORDER BY qa1.id DESC
       LIMIT :page, 10;

NOT EXISTS 变体:

SELECT qa1.*
       FROM questions_and_answers qa1
       WHERE MATCH(qa1.subject, qa1.body) AGAINST (:entry)
             AND qa1.deleted = 0
             AND NOT EXISTS (SELECT *
                                    FROM questions_and_answers qa2
                                    WHERE qa2.id = qa1.related
                                          AND qa2.deleted = 1)
       ORDER BY qa1.id DESC
       LIMIT :page, 10;