計算行之間間隔數

Question

我的表像：

+---------+---------+------------+-----------------------+---------------------+ 
| 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 | 
+---------+---------+------------+-----------------------+---------------------+ 

它記錄遊客的軌跡和對地的一些意見參觀。

我想算在訪問一個特定的地方遊客數量（ID說3227 =）從0:00到23:59，在幾個間隔（即30分鐘）

我試圖做這由：

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 

的問題是，如果沒有記錄，在某些區間下降，這將不會返回時間間隔與次數爲0。例如，有沒有觀衆來自02訪問位置：00至03:00，這不會給我間隔02：00-02：29和02:30 -2：59。

我想要一個確切大小48（每半小時一個）的結果，我該怎麼做？

Answer 1

你必須創建一個表所需的48行，並使用左外連接：

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