mysql - 更新前如何检查记录的值？

Question

我有这张表:

// QandA
+----+-----------+-----------------+-------------+------+------+
| id |    post   | accepted-answer | question_id | let  | type |
+----+-----------+-----------------+-------------+------+------+
| 1  | question1 | null            | 1           | 200  | 0    |
| 2  | answer1   | null            | 1           | null | 1    |
| 3  | answer2   | 1               | 1           | null | 1    |
| 4  | answer3   | null            | 1           | null | 1    |
+----+-----------+-----------------+-------------+------+------+
//                                                        ^ 0 = question | 1 = answer

我还有这个疑问:

// this query changes accepted answer
UPDATE QandA
SET accepted_answer = IF(id <> :id, NULL, 1)
WHERE question_id = :question_id;

<小时/>

现在我尝试在更新之前检查 let 字段。像这样的事情:

if (`let` is null) then {update here}
else {don't update}
where `question_id` = :question_id and `type` = 0

如何在 MySQL 中做到这一点？

Answer 1

您可以使用case语句，下面的语句将更新QandA表。

当let为null并且类型为0时，它会将accepted_answer设置为1。

尝试一下，我知道它适用于 ms sql，只是目前无法访问我的 sql 框。

UPDATE QandA SET accepted_answer =  
CASE
  WHEN let = IS NULL THEN 1
END
WHERE type= 0;