计数和排序 [英] Counting and ordering
本文介绍了计数和排序的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
mysql> describe taps;
+------------+-----------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-----------+------+-----+-------------------+-------+
| tag | int(11) | NO | | NULL | |
| station | int(11) | NO | | NULL | |
| time_Stamp | timestamp | NO | | CURRENT_TIMESTAMP | |
+------------+-----------+------+-----+-------------------+-------+
3 rows in set (0.00 sec)
如何获得每个标签访问的站点数量并按访问次数排序 - 我们只计算对每个站点的一次访问，即使有几次?
How do I get the number of staions visited by each tag and sort by the most visits - where we only count one visit to each staion, even if there were several?
随机数据:
mysql> select * from taps;
+-----+---------+---------------------+
| tag | station | time_Stamp |
+-----+---------+---------------------+
| 50 | 12 | 2016-05-10 20:02:41 |
| 33 | 7 | 2016-05-10 20:02:42 |
| 34 | 13 | 2016-05-10 20:02:42 |
| 31 | 3 | 2016-05-10 20:02:42 |
| 9 | 10 | 2016-05-10 20:02:42 |
| 2 | 11 | 2016-05-10 20:02:42 |
| 33 | 13 | 2016-05-10 20:02:42 |
| 46 | 12 | 2016-05-10 20:02:42 |
| 45 | 1 | 2016-05-10 20:02:42 |
| 34 | 7 | 2016-05-10 20:02:42 |
| 4 | 10 | 2016-05-10 20:08:18 |
| 17 | 5 | 2016-05-10 20:08:19 |
| 46 | 7 | 2016-05-10 20:08:19 |
| 16 | 5 | 2016-05-10 20:08:19 |
| 33 | 3 | 2016-05-10 20:08:19 |
| 31 | 4 | 2016-05-10 20:08:19 |
| 1 | 13 | 2016-05-10 20:08:19 |
| 35 | 2 | 2016-05-10 20:08:19 |
| 9 | 12 | 2016-05-10 20:08:19 |
| 26 | 7 | 2016-05-10 20:08:19 |
+-----+---------+---------------------+
20 rows in set (0.00 sec)
推荐答案
使用 COUNT 函数和 GROUP BY 查找每个标签的访问次数.
Use COUNT funtion and GROUP BY to find the number of visits for each tag.
并使用ORDER BY子句按访问次数降序对结果进行排序.
And use ORDER BY clause to sort the result in descending of the number of visits.
查询
SELECT tag, COUNT(DISTINCT station) AS `no of visits`
FROM taps
GROUP BY tag
ORDER BY COUNT(DISTINCT station) DESC;
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