Question

我的数据库有两张桌子，一张车桌和一张轮桌我试图在几天内找到满足特定条件的车轮数量，但输出中不包含某些天数。

以下是查询：

USE CarDB
SELECT MONTH(c.DateTime1) 'Month',
       DAY(c.DateTime1) 'Day',
       COUNT(w.ID) 'Wheels'
  FROM tblCar c
 INNER JOIN tblWheel w
    ON c.ID = w.CarID
 WHERE c.DateTime1 BETWEEN '05/01/2013' AND '06/04/2013'
   AND w.Measurement < 18
 GROUP BY MONTH(c.DateTime1), DAY(c.DateTime1)
 ORDER BY [ Month ], [ Day ]
GO

输出结果似乎是正确的，但是没有显示0轮的天数。例如：

采样电流输出：

Month Day Wheels
2     1   7
2     2   4
2     3   2 -- 2/4 is missing
2     5   9

所需的输出样本：

Month Day Wheels
2     1   7
2     2   4
2     3   2
2     4   0
2     5   9

我也试过左连接，但似乎没有用。

Answer 1

您可能想要使用LEFT OUTER JOIN：

USE CarDB

SELECT MONTH (c.DateTime1) 'Month', DAY (c.DateTime1) 'Day', COUNT (w.ID) 'Wheels'
FROM tblCar c LEFT OUTER JOIN tblWheel w ON c.ID = w.CarID
WHERE c.DateTime1 BETWEEN '05/01/2013' AND '06/04/2013'
  AND (w.Measurement IS NULL OR w.Measurement < 18)
GROUP BY MONTH (c.DateTime1), DAY (c.DateTime1)
ORDER BY [Month], [Day]
GO

然后，您需要调整WHERE条件，因为您希望由于OUTER连接而使w.Measurement的行保持为NULL。

Answer 2

您使用LEFT JOIN

走在正确的轨道上

尝试使用此类外部联接运行查询，但删除WHERE子句。注意什么？

发生了什么是应用了连接，然后where子句删除了不符合条件的值。所有这一切都发生在小组之前，意味着汽车被排除在外。

以下是您的一种方法：

SELECT Year(cars.datetime1) As the_year
     , Month(cars.datetime1) As the_month
     , Day(cars.datetime1) As the_day
     , Count(wheels.id) As wheels
FROM   (
        SELECT id
             , datetime1
        FROM   tblcar
        WHERE  datetime1 BETWEEN '2013-01-05' AND '2013-04-06'
       ) As cars
 LEFT
  JOIN tblwheels As wheels
    ON wheels.carid = cars.id

这一次的不同之处在于，我们在加入轮桌之前限制了购物车的结果。

Answer 3

删除联接并将您的选择更改为：

SELECT MONTH (c.DateTime1) 'Month', DAY (c.DateTime1) 'Day', isnull(select top 1 (select COUNT from tblWheel where id = tblCar.ID and Measurement < 18), 0)  'Wheels'

错过了一天

3 个答案: