SQL Group By including empty rows
For the sake of this question, let's suppose this table structure:
People:
PersonID int PK
Name varchar(50)
Place int NULL FK -> Places.PlaceID
MovedIn datetime
Places:
PlaceID int PK
Name varchar(50)
I want to determine how many people live at each place:
SELECT pla.PlaceID, COUNT(*)
FROM Places AS pla
LEFT JOIN People as peo ON peo.PlaceID = pla.PlaceID
GROUP BY pla.PlaceID
This query will omit places that have no people living there. Is there any way to make it count 0 instead?
(I'm targetting SQL Server 2005, in the off chance that it matters)
EDIT: Here's my real (anonymized) query, after trying to adapt Steve's solution:
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.Da开发者_运维知识库te) < fc.Days
GROUP BY ft.FooTypeID
(The translation between my initial example and this is: Foo -> People, FooType -> Places, FooConfig -> A third table, for extra fun) I can make this work with Fosco's solution, but I'd prefer Steve's.
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
EDITed question:
Assuming there is always a FooConfig
entry, we'll drop the LEFT JOIN
to that table (as it'll always be there). We can then include the extra criteria in the join to the Foo
table:
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
If the FooConfig
table is optional, then the extra date criteria can't be used (as they would always evaluate to false) - so we'd have to do something like:
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
You can use a column query
select pla.PlaceID, ISNULL((select COUNT(*)
from People
where PlaceID = pla.PlaceID),0) as peopleCount
from Places as pla
order by PlaceID
COUNT() counts the not NULL values. So you can code:
SELECT pla.PlaceID, COUNT(peo.PlaceID) As nbr
FROM Places AS pla
LEFT JOIN People AS peo ON (peo.PlaceID = pla.PlaceID)
DateDiff(day, GetDate(), f.Date) > 0 AND DateDiff(day, GetDate(), f.Date) < fc.Days
turns the LEFT JOINs into an INNER JOINs which isn't what you want.
To keep it a LEFT JOIN add or is null (or put the where clause into the JOIN as Steve Mayne did)
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
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