Question

我有一张这样的表：

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

我尝试为特定用户选择至少15行。只有两个条件：

所有未读的行（seen = NULL）都应该匹配，即使它们超过15行。
如果未读行数超过15，则还应选择2个读取行（seen = 1）。

示例： read是读取行数，unread是notifications表中未读行数。

 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，它不支持第二个条件。

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

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 2

只要选择所有看不见的和（联合）15见。

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和最多15个阅读通知。

之后，如果少于15个看不见，你可以将（客户端）截断为15。

我认为最好的地方就是获取循环只计算看/看不见，并在达到足够行的点处打破循环。

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 3

我找到了解决方案。要添加第二个条件（如果有超过15个未读行，则选择两个读取行），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。第二个得到两行。第三行得到十五行。 UNION会删除重复项，但不会应用其他限制。

Answer 4

请尝试这个，

表T通过time_stamp desc。

返回行号顺序的读取通知

然后从T中选择行＆lt; = GREATEST（15-Count（）未读，2）。

然后将所有未读

联合起来

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 5

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

Answer 6

我可能会简化查询并在应用程序中使用一些后处理逻辑来处理有14或15行未读的边缘情况。只需选择最多17行而不是15行，当您遍历客户端应用程序中的结果集时，除非第14行和第15行未读，否则不必费心检索第16行和第17行。

该查询可以简单如下：

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

如何在MySQL中制作动态限制？

6 个答案: