SQL组通过包含空行

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)

我想确定每个地方住多少人:

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0？

(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

(我最初的例子之间的翻译是:Foo - > People,FooType - > Places,FooConfig - >第三张表,为了更多的乐趣)我可以用Fosco的解决方案来完成这项工作,但我更喜欢Steve的.

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：

假设总是有一个FooConfig条目，我们会将其删除LEFT JOIN到该表中（因为它总是在那里）。然后我们可以在表的连接中包含额外的条件Foo：

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

如果该FooConfig表是可选的，则不能使用额外的日期条件（因为它们总是评估为 false） - 所以我们必须执行以下操作：

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