如何獲得相關行的值？

Question

我有一張這樣的桌子：

// QandA
+----+----------+----------------+------+---------+-----------+
| id | subject  | acceptedanswer | type | related | id_author |
+----+----------+----------------+------+---------+-----------+
| 1  | subject1 | NULL           | 0    | NULL    | 123       |
| 2  |          | 1              | 1    | 1       | 452       |
| 3  | subject2 | NULL           | 0    | NULL    | 635       |
| 4  |          | 1              | 1    | 3       | 432       |
| 5  |          | NULL           | 1    | 1       | 246       |
+----+----------+----------------+------+---------+-----------+

/* columns explanations:
- acceptedanswer can be NULL, 0, 1. (0 and NULL are identical). (it is NULL for questions)
- type is 0 for questions and 1 for answers.
- related is NULL for questions. For answers is containing the number of its own question
*/

好吧，我有兩個參數：

• 答案的id號： $id = 5
• id_author的那個答案$id_author = 246

現在，我正在嘗試獲得以下兩點：

• 欠費問題的主題
• 指定有自己的問題是否接受。

這是我的查詢：

SELECT t2.subject
FROM qanda t1
JOIN qanda t2 ON t1.related = t2.id AND t2.type = 0
WHERE t1.id = $id AND t2.id_author = $id_author

當前結果：

+----------+
| subject1 |
+----------+

預期結果：

+----------+---+
| subject1 | 1 |
+----------+---+
--           ^ this 1 means the question of this answer has a accepted answer
--           ^ this should be 0 if there isn't any accepted answer for that question

我怎樣才能做到這一點？

Answer 1

嘗試這個：

SELECT t2.subject, max(COALESCE(t3.accepted, 0))
FROM qanda t1
INNER JOIN qanda t2 ON t1.related = t2.id AND t2.type = 0
INNER JOIN qanda t3 ON t1.related = t3.related
WHERE t1.id = $id AND t2.id_author = $id_author
GROUP BY t3.related

不過，我不確定我是否理解您的意圖。 COALESCE消除null，並使用max()來查看是否已接受任何答案。 我假設只有答案在related列中具有非空值。

另外，也許最好不要在表中保留不同的內容？

Answer 2

如果您想知道答案id = 5回答的問題是否具有可接受的答案，並假設（如注釋中所述）每個問題只能有一個可接受的答案，那么類似這樣的事情（請注意：兩個聯接）應該起作用。 ..

SELECT x.subject
     , y.id IS NOT NULL has_accepted
  FROM qanda x
  LEFT
  JOIN qanda y
    ON y.related = x.id
   AND y.type = 1
   AND y.acceptedanswer = 1
  JOIN qanda z
    ON z.related = x.id
 WHERE x.type = 0 
   AND z.id = 5;