Question

为了这个问题，让我们假设这个表结构：

People:
PersonID int PK
Name varchar(50)
Place int NULL FK -> Places.PlaceID
MovedIn datetime

Places:
PlaceID int PK
Name varchar(50)

我想确定每个地方有多少人住在这里：

SELECT pla.PlaceID, COUNT(*)
FROM Places AS pla
LEFT JOIN People as peo ON peo.PlaceID = pla.PlaceID
GROUP BY pla.PlaceID

此查询将省略没有人居住的地方。有没有办法让它算上0呢？

（我的目标是SQL Server 2005，重要的是它很重要）

编辑：在尝试调整Steve的解决方案之后，这是我的真实（匿名）查询：

SELECT
    ft.FooTypeID, COUNT(f.FooID)
FROM FooType as ft
LEFT OUTER JOIN Foo f ON ft.FooTypeID = f.FooTypeID
LEFT JOIN FooConfig fc ON ft.NotificationConfigID = fc.FooConfigID
WHERE
    DateDiff(day, GetDate(), f.Date) > 0 AND
    DateDiff(day, GetDate(), f.Date) < fc.Days
GROUP BY ft.FooTypeID

（我的初始示例与此之间的转换是：Foo - ＆gt; People，FooType - ＆gt; Places，FooConfig - ＆gt;第三个表，为了额外的乐趣）我可以用Fosco的解决方案来完成这项工作，但我更喜欢史蒂夫的解决方案。

Answer 1

SELECT pla.PlaceID, COUNT(peo.PersonID)
FROM Places AS pla LEFT OUTER JOIN People as peo ON peo.PlaceID = pla.PlaceID
GROUP BY pla.PlaceID

编辑问题：

假设总有一个FooConfig条目，我们会将LEFT JOIN放到该表中（因为它始终存在）。然后，我们可以在联接中将额外条件包括在Foo表中：

SELECT
    ft.FooTypeID, COUNT(f.FooID)
FROM FooType as ft
  JOIN FooConfig fc ON ft.NotificationConfigID = fc.FooConfigID
  LEFT OUTER JOIN Foo f ON ft.FooTypeID = f.FooTypeID AND
    DateDiff(day, GetDate(), f.Date) > 0 AND
    DateDiff(day, GetDate(), f.Date) < fc.Days
GROUP BY ft.FooTypeID

如果FooConfig表是可选的，那么不能使用额外的日期标准（因为它们总是评估为假） - 所以我们必须做类似的事情：

SELECT
    ft.FooTypeID, COUNT(f.FooID)
FROM FooType as ft
  LEFT OUTER JOIN FooConfig fc ON ft.NotificationConfigID = fc.FooConfigID
  LEFT OUTER JOIN Foo f ON ft.FooTypeID = f.FooTypeID AND
    (
      (DateDiff(day, GetDate(), f.Date) > 0 AND
       DateDiff(day, GetDate(), f.Date) < fc.Days) 
      OR
      (fc.Days IS NULL)
    )
GROUP BY ft.FooTypeID

Answer 2

您可以使用列查询

select pla.PlaceID, ISNULL((select COUNT(*) 
                             from People 
                             where PlaceID = pla.PlaceID),0) as peopleCount
from Places as pla
order by PlaceID

Answer 3

COUNT（）计算非NULL值。所以你可以编码：

SELECT pla.PlaceID, COUNT(peo.PlaceID) As nbr
FROM Places AS pla
LEFT JOIN People AS peo ON (peo.PlaceID = pla.PlaceID)

Answer 4

DateDiff(day, GetDate(), f.Date) > 0 AND DateDiff(day, GetDate(), f.Date) < fc.Days将LEFT JOIN转换为INNER JOIN，这不是你想要的。

要保持LEFT JOIN的添加或为空（或者像Steve Mayne那样将where子句放入JOIN中）

SELECT
    ft.FooTypeID, COUNT(f.FooID)
FROM FooType as ft
LEFT OUTER JOIN Foo f ON st.FooTypeID = s.FooTypeID
LEFT JOIN FooConfig fc ON st.NotificationConfigID = fc.FooConfigID
WHERE
    (DateDiff(day, GetDate(), f.Date) > 0 
        or f.Date IS NULL)
    AND 
   (DateDiff(day, GetDate(), f.Date) < fc.Days
         or fc.Days Is NULLL or f.Date Is NULL )
GROUP BY ft.FooTypeID

SQL组通过包含空行

4 个答案: