如何在MySQL中进行动态限制？

Question

我有一个 table 这样的：

// 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 |
+----+--------------+------+---------+------------+

我正在尝试为特定用户 select 至少 15 行。只是有两个条件：

始终应匹配所有未读行 (seen = NULL)，即使它们超过 15 行。
如果未读行数大于15，那么也应该select 2个已读行(seen = 1).

例子： read是已读行数，unread是未读行数 notifications table.

 read | unread |          output should be           
------|--------|-------------------------------------
 3    | 8      | 11 rows                             
 12   | 5      | 15 rows (5 unread, 10 read)         
 20   | 30     | 32 rows (30 unread, 2 read)         
 10   | 0      | 10 rows (0 unread, 10 read)         
 10   | 1      | 11 rows (1 unread, 10 read)         
 10   | 6      | 15 rows (6 unread, 9 read)          
 100  | 3      | 15 rows (3 unread, 12 read)         
 3    | 100    | 102 rows (100 unread, 2 read)

这是我当前的查询，不支持第二个条件。

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 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;

Answer 1

我可能会简化查询并在应用程序中使用一些 post 处理逻辑来处理有 14 或 15 行未读的边缘情况。只需 select 最多 17 行而不是 15 行，并且当您在客户端应用程序中循环遍历结果集时，只要不费心检索第 16 行和第 17 行，除非第 14 行和/或第 15 行未读。

该查询可以很简单：

SELECT id, event, seen, time_stamp 
FROM notifications n
WHERE id_user = :id
ORDER BY seen DESC, time_stamp DESC
LIMIT 17

Answer 2

SELECT id, event, seen, time_stamp 
 FROM notifications n
 WHERE id_user = 123 AND seen IS NULL

UNION

(SELECT id, event, seen, time_stamp 
FROM ( 
 SELECT id, event, seen, n.id_user, time_stamp, un.CNT
    FROM notifications n
    JOIN (
        SELECT COUNT(1) CNT, id_user
        FROM notifications
        WHERE id_user = 123 and seen is NULL
        group by id_user
        ) un
    ON n.id_user = un.id_user
    WHERE CNT > 15
) t1
WHERE t1.SEEN is not NULL
LIMIT 2)

UNION

SELECT id, event, seen, time_stamp 
FROM ( 
 SELECT id, event, seen, n.id_user, time_stamp, un.CNT
    FROM notifications n
    JOIN (
        SELECT COUNT(1) CNT, id_user
        FROM notifications
        WHERE id_user = 123 and seen is NULL
        group by id_user
        ) un
    ON n.id_user = un.id_user
    WHERE CNT < 15
) t1
WHERE t1.SEEN is not NULL

Answer 3

只有 select 所有未见过的和（与）15 个见过的。

SELECT id, event, seen, time_stamp 
 FROM notifications n
 WHERE id_user = :id AND seen IS NULL
UNION ALL
(SELECT id, event, seen, time_stamp 
 FROM notifications n
 WHERE id_user = :id AND seen IS NOT NULL
 LIMIT 15)

因此，您现在拥有所有未读通知和最多 15 条已读通知。

之后，如果看不见的少于 15 个，您可以将（客户端）截断为 15 个。

我认为最好的地方是获取循环。
只需计数 seen/unseen 并在达到足够行数时打破循环。

一些伪代码php:

$read = $unread = 0;

while($row = $db->fetch()) {
  if ($row['seen']) $read++;
  if (!$row['seen']) $unread++;
  // ...
  if ($weHaveEnoughRows) break;
}

Answer 4

我找到了解决办法。要添加第二个条件 （如果有超过 15 个未读行，则选择两个已读行），我必须再使用一个 UNION。像这样：

(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;

第一个子查询获取所有看不见的行。第二个得到两个可见的行。第三个有十五行。 UNION 删除重复项，但不应用其他限制。

Answer 5

请试试这个，

table T returns 读取行号按 time_stamp desc.

排序的通知

然后你 select 从 T 行 <= GREATEST(15-Count() of unread,2).

然后将所有与未读合并

SELECT id,event,seen,time_stamp 
FROM 
  (SELECT id, event, seen, time_stamp,@row:=@row+1 as row 
   FROM notifications n,(SELECT @row := 0)r
   WHERE id_user = :id AND seen IS NOT NULL
   ORDER BY time_stamp desc
   )T
WHERE T.row <= GREATEST(15-
                   (SELECT COUNT(*) FROM notifications n
                    WHERE id_user = :id AND seen IS NULL),2)
UNION ALL
(SELECT id, event, seen, time_stamp 
 FROM notifications n
 WHERE id_user = :id
 AND seen is NULL
)
ORDER BY (seen IS NULL) desc,time_stamp desc

Answer 6

尝试：

SET @`id_user` := 123;

SELECT `id`, `event`, `seen`, `time_stamp`
FROM (SELECT `id`, `event`, `seen`, `time_stamp`, @`unread` := @`unread` + 1
      FROM `notifications`, (SELECT @`unread` := 0) `unr`
      WHERE `id_user` = @`id_user` AND `seen` IS NULL
      UNION ALL
      SELECT `id`, `event`, `seen`, `time_stamp`, @`read` := @`read` + 1
      FROM `notifications`, (SELECT @`read` := 0) `r`
      WHERE `id_user` = @`id_user` AND `seen` IS NOT NULL
            AND (
                 @`read` < (15 - @`unread`) OR
                 ((15 - @`unread`) < 0 AND @`read` < 2)
            )
) `source`;

SQL Fiddle demo

如何在MySQL中进行动态限制？

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