Question

我有一张这样的表：

// QandA
+----+----------------------------------------+---------+----------+-----------+
| Id |                   body                 | related | accepted | author_id |
+----+----------------------------------------+---------+----------+-----------+
| 1  | content of question1                   | null    | null     | 12345     |
| 2  | content of first answer for question1  | 1       | 0        | 53456     |
| 3  | content of question2                   | null    | null     | 43634     |
| 4  | content of second answer for question1 | 1       | 0        | 43665     |
| 5  | content of first answer for question2  | 3       | 1        | 43324     |
+----+----------------------------------------+---------+----------+-----------+

/* related column: Actually that's just for answers and this column is containing the id of
   its question. (questions always are null) */

/* accepted column: Actually that's just for answers and specifics accepted answer.
   0 means it isn't accepted answer, 1 means it is accepted answer.
   (questions always are null) */

我在设置问题的接受答案之前尝试实施此条件：

条件：验证当前用户是否为OP。 author_id of其问题应与$_SESSION['id']相同。

以下是我的查询：（我拥有的所有数据都只是已接受答案的ID :answer_id）

UPDATE QandA q CROSS JOIN ( SELECT related FROM QandA WHERE id = :answer_id ) x
    SET accepted = ( id = :answer_id ) -- this acts like a if statement 
  WHERE q.related   = x.related
        AND
        -- validating OP
        (SELECT 1 FROM QandA 
          WHERE id = x.related AND
                author_id = $_SESSION['id']
        )

＃1093 - 您无法指定目标表＆＃39; tbname＆＃39;用于FROM子句中的更新

我该如何解决？

编辑：实际上还有一个条件：

+----+----------------------------------------+---------+----------+-----------+------+
| Id |                   body                 | related | accepted | author_id | free |
+----+----------------------------------------+---------+----------+-----------+------+
| 1  | content of question1                   | null    | null     | 12345     | null |
| 2  | content of first answer for question1  | 1       | 0        | 53456     | null |
| 3  | content of question2                   | null    | null     | 43634     | 300  |
| 4  | content of second answer for question1 | 1       | 0        | 43665     | null |
| 5  | content of first answer for question2  | 3       | 1        | 43324     | null |
+----+----------------------------------------+---------+----------+-----------+------+

/* free column: Actually that's just for questions. `null` means it is a free question
   and any number else means it isn't. (answers always are `null`) */

附加条件：如果问题是免费的，那么OP可以接受答案，并更改他接受的答案，并撤消他接受的答案。但如果问题不是免费的，那么OP就可以接受一次问题，而且他无法撤消它，他也无法改变已接受的答案。这是在MySQL中实现该条件：

(SELECT 1 FROM QandA
  WHERE id = x.related AND
        (
          ( free IS NOT NULL AND
            NOT IN ( SELECT 1 FROM QandA
                      WHERE related = x.related AND
                            accepted = 1 )
          ) OR free IS NULL
        )
)

Answer 1

我认为应该这样做：

UPDATE QandA AS ans1
JOIN QandA AS ans2 ON ans2.related = ans1.related
JOIN QandA AS ques ON ans2.related = ques.id
SET ans1.accepted = (ans1.id = :answer_id)
WHERE ques.author_id = :session_id
AND ans2.id = :answer_id

第一个JOIN过滤到与接受答案相同的问题的答案。

第二个JOIN发现了这个问题。

WHERE子句将更新仅限于给定作者的问题，并指定接受的答案ID。

DEMO

对于附加条件，请添加

AND (ques.free IS NULL or ans1.accepted IS NULL)

到WHERE子句。 ques.free IS NULL匹配任何免费问题，ans1.accepted IS NULL匹配没有接受答案的问题（因为当答案被接受时，该问题的所有其他答案都会获得accepted = 0）。

DEMO of question with no accepted answer
DEMO of question that's free

如何在UPDATE语句中使用JOIN？

1 个答案: