我有一个这样的表：

// 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) */

在设置问题的可接受答案之前，我正在尝试实现此 条件 ：

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

这是我的查询：（ 我拥有的所有数据只是接受的答案的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-您无法在FROM子句中指定目标表’tbname’进行更新

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`) */

其他条件：
如果问题是免费的，则OP可以接受该问题的答案，然后更改其接受的答案，然后撤消其接受的答案。但是，如果问题不是免费的，那么OP可以一次接受一个问题，他不能撤消它，也不能更改已接受的答案。这是在MySQL中实现该条件的方法：

(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
        )
)