在MySQL查询中使用SUM和IF时，值计数错误

当将IF函数与sum结合使用时，我的数据库查询中存在计数问题

具有相同行为的示例数据库：

CREATE TABLE `test` (
 `id` int(11) NOT NULL,
  `system` varchar(11) NOT NULL,
  `status` varchar(11) NOT NULL,
  `timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

示例数据：

INSERT INTO `test` (`id`, `system`, `status`, `timestamp`) VALUES
(3, 'systema', 'ok', '2017-03-25 06:43:03'),
(4, 'systema', 'ok', '2017-03-25 06:43:09'),
(5, 'systema', 'fail', '2017-03-25 06:43:16'),
(6, 'systema', 'ok', '2017-03-25 06:43:22'),
(7, 'systema', 'ok', '2017-03-25 06:43:31'),
(8, 'systema', 'fail', '2017-03-25 07:12:28'),
(9, 'systema', 'fail', '2017-03-25 07:13:07'),
(10, 'systema', 'ok', '2017-03-25 07:13:13'),
(11, 'systema', 'ok', '2017-03-25 07:26:21'),
(12, 'systema', 'fail', '2017-03-25 07:26:41'),
(13, 'systema', 'fail', '2017-03-25 07:28:52'),
(14, 'systema', 'fail', '2017-03-25 07:29:13'),
(15, 'systema', 'fail', '2017-03-25 07:29:39');

确保所有变量均为空：

mysql> set @prev_system:=null;
Query OK, 0 rows affected (0.00 sec)

mysql> set @prev_status:=null;
Query OK, 0 rows affected (0.00 sec)

mysql> set @prev_time:=null;
Query OK, 0 rows affected (0.00 sec)

现在，我们进行数据库查询，不进行计数：

mysql> select @prev_system as old_system, @prev_status as old_status, @prev_time 
as old_time, if (@prev_status <> status and @prev_system = system and @prev_time 
< timestamp,1,NULL) as hops, @prev_system:=system as system, @prev_status:=status as 
status, @prev_time:=timestamp as timestamp from test;

与旧状态相比，状态已更改时，列中的“跃点”应为“1”，如果未发生更改，则应为“NULL”，到目前为止，情况良好

但是，当我将SUM语句添加到IF语句时，在hops列中没有得到预期的“5”作为exprected结果：

查询：

mysql> select @prev_system as old_system, @prev_status as old_status, @prev_time 
as old_time, sum(if (@prev_status <> status and @prev_system = system and @prev_time < timestamp,1,NULL)) as hops, @prev_system:=system as system, 
@prev_status:=status as status, @prev_time:=timestamp as timestamp from test;

我想我错过了一件很小的事情，谁能帮我把总数算对呢

---

谢谢

您没有以任何方式对它们进行排序，它们可能以任何顺序出现，这可能会影响到某些内容。我忘了添加它们，但这对结果没有影响，我按系统添加了顺序，在查询末尾添加了时间戳，结果和输出是相同的，结果仍然是“7”，正如您在前面的输出中所看到的，dataorder似乎没有问题，但是结果是出乎意料的是，后一种顺序并不重要，因为只有一行。你可能需要一个子查询。我正试图阻止子查询，因为真实的数据集非常大，而且在增长，而且子查询速度很慢。我希望任何人都能帮助我找到一个解决方案，因为我对这种增强型mysql没有太多经验。谢谢你，萨米。

mysql> select @prev_system as old_system, @prev_status as old_status, @prev_time 
as old_time, sum(if (@prev_status <> status and @prev_system = system and @prev_time < timestamp,1,NULL)) as hops, @prev_system:=system as system, 
@prev_status:=status as status, @prev_time:=timestamp as timestamp from test;

+------------+------------+----------+------+---------+--------+---------------------+
| old_system | old_status | old_time | hops | system  | status | timestamp           |
+------------+------------+----------+------+---------+--------+---------------------+
| NULL       | NULL       | NULL     |    7 | systema | ok     | 2017-03-25 07:43:03 |
+------------+------------+----------+------+---------+--------+---------------------+