Question

我的表就像：

+---------+---------+------------+-----------------------+---------------------+
| visitId | userId  | locationId | comments              | time                |
+---------+---------+------------+-----------------------+---------------------+
|       1 |    3    |     12     | It's a good day here! | 2012-12-12 11:50:12 |
+---------+---------+------------+-----------------------+---------------------+
|       2 |    3    |     23     | very beautiful        | 2012-12-12 12:50:12 |
+---------+---------+------------+-----------------------+---------------------+
|       3 |    3    |     52     | nice                  | 2012-12-12 13:50:12 |
+---------+---------+------------+-----------------------+---------------------+

记录访客的轨迹以及对所访问地点的一些评论。

我想计算从0:00到23:59，在某个时间间隔（即30分钟）内访问特定地点（比如id = 3227）的访客数量

我试图通过以下方式执行此操作

SELECT COUNT(*) FROM visits 
GROUP BY HOUR(time), SIGN( MINUTE(time) - 30 )// if they are in the same interval this will yield the same result
WHERE locationId=3227

问题是如果没有记录落在某个时间间隔内，则不会返回该计数为0的时间间隔。例如，没有访问者从02:00到该位置访问03:00，这不会给我02：00-02：29和02：30-2：59的间隔。

我想要一个精确大小为48 的结果（每半小时一个），我该怎么做？

Answer 1

您必须创建一个包含所需48行的表，并使用左外连接：

select n.hr, n.hr, coalesce(v.cnt, 0) as cnt
from (select 0 as hr, -1 as sign union all
      select 0, 1 union all
      select 1, -1 union all
      select 1, 1 union all
      . . .
      select 23, -1 union all
      select 23, 1 union all
     ) left outer join
     (SELECT HOUR(time) as hr, SIGN( MINUTE(time) - 30 ) as sign, COUNT(*) as cnt
      FROM visits 
      WHERE locationId=3227
      GROUP BY HOUR(time), SIGN( MINUTE(time) - 30 )
     ) v
     on n.hr = v.hr and n.sign = v.sign
order by n.hr, n.hr

计算间隔之间的行数

1 个答案: