php - 如何更新 SELECT 语句的结果？

Question

我有一个这样的表:

// notifications
+----+--------------+------+---------+------------+
| id |      event   | seen | id_user | time_stamp |
+----+--------------+------+---------+------------+
| 1  | vote         | 1    | 123     | 1464174617 |
| 2  | comment      | 1    | 456     | 1464174664 |
| 3  | vote         | 1    | 123     | 1464174725 |
| 4  | answer       | 1    | 123     | 1464174813 |
| 5  | comment      | NULL | 456     | 1464174928 |
| 6  | comment      | 1    | 123     | 1464175114 |
| 7  | vote         | NULL | 456     | 1464175317 |
| 8  | answer       | NULL | 123     | 1464175279 |
| 9  | vote         | NULL | 123     | 1464176618 |
+----+--------------+------+---------+------------+ 

这是我的查询:

(SELECT id, event, seen, time_stamp 
 FROM notifications n
 WHERE id_user = :id AND seen IS NULL
)UNION
(SELECT id, event, seen, time_stamp
 FROM notification n
 WHERE id_user = :id AND seen IS NOT NULL
 LIMIT 2
)UNION 
(SELECT id, event, seen, time_stamp 
 FROM notifications n
 WHERE id_user = :id 
 ORDER BY (seen IS NULL) desc, time_stamp desc
 LIMIT 15
)
ORDER BY (seen IS NULL) desc, time_stamp desc;

---

现在我正在尝试从上面的查询更新匹配的行并设置 seen = 1，像这样:

UPDATE notifications SET seen = 1 WHERE /* the result of query above */

我该怎么做？

---

注意:我还使用 PHP 和 PDO 来执行该 SELECT 查询(如果知道这一点很重要)

$stm = $db->prepare(" (SELECT id, ... ");
$stm->execute( /* passing some variables */);
$stm->fetchAll(PDO::FETCH_ASSOC);

Answer 1

seen 只能是1 或null。在这种情况下，您从该 UNION 查询中获得的大部分内容都是无关紧要的。在我看来，您尝试进行的更新的最终结果将是:

UPDATE notifications SET seen = 1 WHERE id_user = :id AND seen IS NULL

任何其他东西都会将已经是 1 的内容更新为 1。