Php 在mySQL中基于COUNT（）值限制分组

我正在将事件记录到一个mySQL数据库中，并希望获得前3个事件，以便进行监视

我的表事件日志如下所示：

+----+------------------+---------------------+
| id |    eventname     |      eventdate      |
+----+------------------+---------------------+
|  0 | machine1.started | 2016-09-04 19:22:23 |
|  1 | machine2.reboot  | 2016-09-04 20:23:11 |
|  2 | machine1.stopped | 2016-09-04 20:24:12 |
|  3 | machine1.started | 2016-09-04 20:25:12 |
|  4 | machine1.stopped | 2016-09-04 23:23:16 |
|  5 | machine0.started | 2016-09-04 23:24:00 |
|  6 | machine1.started | 2016-09-04 23:24:16 |
|  7 | machine3.started | 2016-09-04 23:25:00 |
|  8 | machine4.started | 2016-09-04 23:26:00 |
|  9 | cluster.alive    | 2016-09-04 23:30:00 |
| 10 | cluster.alive    | 2016-09-05 11:30:00 |
+----+------------------+---------------------+

查询最终应返回以下内容，保留

根据mySQL的COUNT函数生成的列eventcounts，最常发生的前3个事件按其eventname分组 只有eventcount=1的2行，但仅当1在前3个eventcounts内时，因为有很多事件刚刚发生 曾经，因此会使我的前端过载 基于上表的预期结果示例：

+------------+------------------+
| eventcount |    eventname     |
+------------+------------------+
|          3 | machine1.started |
|          2 | machine1.stopped |
|          2 | cluster.alive    |
|          1 | machine0.started |
|          1 | machine2.started |
+------------+------------------+

请注意，我不需要仅返回3行，而是需要具有3个最高事件计数的行

我用下面的查询字符串做了很多实验，包括多次选择和可疑案例。。。当条件允许时，但无法使其按我需要的方式工作

SELECT COUNT(id) AS 'eventcount', eventname
FROM eventlog
GROUP BY eventname
ORDER BY eventcount DESC;

---

什么是以高效的方式获得所需结果的最佳方法？

MySQL中的这些类型的条件是痛苦的。一种方法使用变量。这里有一种方法不需要：

SELECT el.eventcount, el.eventname
FROM (SELECT COUNT(el.id) AS eventcount, el.eventname
      FROM eventlog el
      GROUP BY el.eventname
     ) el JOIN
     (SELECT cnt
      FROM (SELECT DISTINCT COUNT(el.id) as cnt
            FROM eventlog el
            GROUP BY el.eventname
           ) el
      ORDER BY cnt DESC
      LIMIT 3
     ) ell
     ON ell.cnt = el.eventcount
ORDER BY el.eventcount DESC;

编辑：

使用变量的解决方案如下所示，包括计数为1时2的限制：

SELECT *
FROM (SELECT e.*,
             (@rn1 := if(@c1 = eventcount, @rn1 + 1,
                         if(@c1 := eventcount, 1, 1)
                        )
             ) as rn
      FROM (SELECT e.*,
                   (@rn := if(@c = eventcount, @rn,
                              if(@c := eventcount, @rn + 1, @rn + 1)
                             )
                   ) as rank
            FROM (SELECT COUNT(el.id) AS eventcount, el.eventname
                  FROM eventlog el
                  GROUP BY el.eventname
                 ) e CROSS JOIN
                 (SELECT @c := 0, @rn := 0) params
            ORDER BY eventcount DESC
           ) e CROSS JOIN
           (SELECT @c1 := 0, @rn1 := 0) params
      ORDER BY eventcount DESC
     ) e
WHERE rank <= 3 AND
      (eventcount > 1 OR rn <= 2);

---

最里面的计数枚举计数。第二个在计数内枚举。实际上，这两个查询可以组合成一个子查询，但要小心。

您可以尝试以下方法：

SELECT count(eventname), eventname FROM table
group by eventname
HAVING(count(eventname)) > 1
order by count(eventname) DESC
limit 3

---

这里有一种使用变量的方法 SQL Fiddle for it：

---

这应该可以重构一点，但它现在返回了正确的答案：

SELECT eventcount, eventname
FROM
(SELECT el.eventcount, el.eventname
FROM (SELECT COUNT(el.id) AS eventcount, el.eventname
      FROM eventlog el
      GROUP BY el.eventname
     ) el JOIN
     (SELECT counts
      FROM (SELECT DISTINCT COUNT(el.id) as counts
            FROM eventlog el
            GROUP BY el.eventname
           ) el
      ORDER BY counts DESC
      LIMIT 3
     ) el2
     ON el2.counts = el.eventcount
     WHERE el.eventcount != 1
UNION ALL
(SELECT el.eventcount, el.eventname
FROM (SELECT COUNT(el.id) AS eventcount, el.eventname
      FROM eventlog el
      GROUP BY el.eventname
     ) el JOIN
     (SELECT counts
      FROM (SELECT DISTINCT COUNT(el.id) as counts
            FROM eventlog el
            GROUP BY el.eventname
           ) el
      ORDER BY counts DESC
      LIMIT 3
     ) el2
     ON el2.counts = el.eventcount AND el2.counts = 1
     LIMIT 2)) tmp
ORDER BY tmp.eventcount DESC;

---

SQL FIDLE:

如果可以使用临时表

预先计算事件计数并将结果存储在临时表中：

create temporary table tmp_eventcounts
  select eventname, count(1) as eventcount
  from eventlog
  group by eventname
  order by eventcount desc
;

create temporary table tmp_top3counts
  select distinct eventcount
  from tmp_eventcounts
  order by eventcount desc
  limit 3
;

tmp_事件计数的内容：

选择前3个事件计数并将其存储在另一个临时表中：

create temporary table tmp_eventcounts
  select eventname, count(1) as eventcount
  from eventlog
  group by eventname
  order by eventcount desc
;

create temporary table tmp_top3counts
  select distinct eventcount
  from tmp_eventcounts
  order by eventcount desc
  limit 3
;

tmp_Top3计数的内容：

现在选择eventcount前3名但eventcount>1的所有EventName。 还可以选择最多两个具有前3个eventcounts但eventcount=1的EventName。 使用UNION组合两个结果：

select eventcount, eventname
from tmp_top3counts
join tmp_eventcounts using(eventcount)
where eventcount > 1
union all (
  select eventcount, eventname
  from tmp_top3counts
  join tmp_eventcounts using(eventcount)
  where eventcount = 1
  limit 2
)
order by eventcount desc;

结果:

| eventcount |        eventname |
|------------|------------------|
|          3 | machine1.started |
|          2 | machine1.stopped |
|          2 |    cluster.alive |
|          1 |  machine2.reboot |
|          1 | machine3.started |

如果不能使用临时表，则可以使用临时表的定义替换临时表的引用，并创建一个高度不可读但有效的查询：

select eventcount, eventname
from (
  select distinct eventcount
  from (
    select eventname, count(1) as eventcount
    from eventlog
    group by eventname
  ) tmp_eventcounts
  order by eventcount desc
  limit 3  
) tmp_top3counts
join (
  select eventname, count(1) as eventcount
  from eventlog
  group by eventname
) tmp_eventcounts using(eventcount)
where eventcount > 1
union all (
  select eventcount, eventname
  from (
    select distinct eventcount
    from (
      select eventname, count(1) as eventcount
      from eventlog
      group by eventname
    ) tmp_eventcounts
    order by eventcount desc
    limit 3
  ) tmp_top3counts
  join (
    select eventname, count(1) as eventcount
    from eventlog
    group by eventname
  ) tmp_eventcounts using(eventcount)
  where eventcount = 1
  limit 2
)
order by eventcount desc;

)-

虽然这看起来很疯狂，但可以在PHP中轻松创建：

$tmp_eventcounts = "
    select eventname, count(1) as eventcount
    from eventlog
    group by eventname
";

$tmp_top3counts = "
    select distinct eventcount
    from ( {$tmp_eventcounts} ) tmp_eventcounts
    order by eventcount desc
    limit 3
";

$sql = "
    select eventcount, eventname
    from ( {$tmp_top3counts} )  tmp_top3counts
    join ( {$tmp_eventcounts} ) tmp_eventcounts using(eventcount)
    where eventcount > 1
    union all (
      select eventcount, eventname
      from ( {$tmp_top3counts} )  tmp_top3counts
      join ( {$tmp_eventcounts} ) tmp_eventcounts using(eventcount)
      where eventcount = 1
      limit 2
    )
    order by eventcount desc
";

---

注意：看起来MySQL需要一次又一次地执行相同的子查询。但是它应该能够缓存结果并重用它们。

@riggsfully不，它不会。样本数据的预期结果为5rows@Lamak好的观点这将正确地进行排名，但是如果eventcount=1在前3个结果中，OP只想返回其中的2个记录，这将不会限制这一点，并将返回测试用例中的所有4个记录above@Matt是的，你说得对，排名很好，它似乎运行效率很高，但不考虑事件计数的限制= 1……不同的计数组？呃？对于任何想知道的人来说：@Gordon Linoff和Matt的解决方案对我来说都非常有效。我接受了这个答案，因为查询更快完成。

$tmp_eventcounts = "
    select eventname, count(1) as eventcount
    from eventlog
    group by eventname
";

$tmp_top3counts = "
    select distinct eventcount
    from ( {$tmp_eventcounts} ) tmp_eventcounts
    order by eventcount desc
    limit 3
";

$sql = "
    select eventcount, eventname
    from ( {$tmp_top3counts} )  tmp_top3counts
    join ( {$tmp_eventcounts} ) tmp_eventcounts using(eventcount)
    where eventcount > 1
    union all (
      select eventcount, eventname
      from ( {$tmp_top3counts} )  tmp_top3counts
      join ( {$tmp_eventcounts} ) tmp_eventcounts using(eventcount)
      where eventcount = 1
      limit 2
    )
    order by eventcount desc
";