如何在同一查询中同时使用ON DUPLICATE KEY和REPLACE INTO

我有一个这样的表：

// AcceptedAnswer
+----+---------+-------------+-----------+
| id | user_id | question_id | answer_id |
+----+---------+-------------+-----------+
| 1  | 123     | 4335345     | 3342434   |
| 2  | 345     | 4565546     | 3443565   |
+----+---------+-------------+-----------+
// user_id : the id of that person who is author of question

此外，我在这三列上有一个唯一索引组：(user_id, question_id, answer_id)。

有三种情况：

• 当没有任何可接受的答案并且OP（user_id）想要接受asnwer时
• 当有一个已接受的答案并且OP想要撤消它时
• 当有一个已接受的答案，并且OP希望将其更改为另一个答案时

我的剧本也适用于前两种情况。但这对第三种情况不起作用。我该如何实现？

这是我的脚本：

try {
    // DB connection here
    // This SELECT statement validates whether user_id is the author of the question     
    $stmt = $db_con->prepare("INSERT INTO AcceptedAnswer(user_id, question_id, answer_id)
                              SELECT ?,?,?
                              FROM questions q
                              WHERE q.id = ? and q.author_id = ? limit 1;");
    $stmt->execute( array( $_SESSION["Id"], $question_id, $answer_id,
                           $question_id, $_SESSION["Id"] ) );

} catch(PDOException $e) {
    // undo acceptance
    if ((int) $e->getCode() === 23000) {
    $stmt = $dbh_conn->prepare(" DELETE FROM AcceptedAnswer
                                 WHERE user_id = ? AND
                                       question_id = ? AND
                                       answer_id = ? ");
    $stmt->execute(array($_SESSION["Id"], $question_id, $answer_id));
}

例如：（基于上表）

当我通过这个：(123, 4335345, 2353423)（更改接受的答案）时，预期输出：

// AcceptedAnswer
+----+---------+-------------+-----------+
| id | user_id | question_id | answer_id |
+----+---------+-------------+-----------+
| 1  | 123     | 4335345     | 2353423   |
| 2  | 345     | 4565546     | 3443565   |
+----+---------+-------------+-----------+

当我通过这个：(345, 4565546, 3443565)（撤消接受的答案）时，预期输出：

// AcceptedAnswer
+----+---------+-------------+-----------+
| id | user_id | question_id | answer_id |
+----+---------+-------------+-----------+
| 1  | 123     | 4335345     | 2353423   |
+----+---------+-------------+-----------+