SQL - LEFT OUTER JOIN and WHERE clause

Question

Hello,

I'm terrible at SQL. I do not know if what I am trying to do is possible. But, because of our data structure, I need to solve this problem this way or do a massive architectural change.

I am trying to count the number of 'Provinces' (a.k.a States) for a Country. However, there are just a few Provinces that need to be ignored from the count. Because of this, I am trying to retrieve a list of countries, with a count of the provinces in each country.

As an example, I need to query for the United States, and ignore 'Washington D.C.' from the count. The reason why is because by our requirements, Washington D.C. is not a state. Here is what I am trying at the moment (it does not work):

SELECT
  c.Name AS 'CountryName',
  ISNULL(COUNT(p.[ID]), 0) as 'ProvinceCount'
FROM 
  Country c LEFT OUTER JOIN [Province] p ON p.[CountryID]=c.[ID]
WHERE
  c.[ID]=@idParameter and
  p.[Name] <> 'Washington D.C.'

As you can imagine, this query does not return any results when the idParameter matches that of the United States.

How do I get the correct count while figuring in exceptions? Thank you very much for your help.

Answer 1

select
  c.name as 'country name'
  isnull(count(p.[ID]), 0) as 'provice count'
from
  Country c
inner join
  Province p on
  p.CountryID = c.[ID]
where
  c.[ID] = @idParameter
  and
  p.[Name] not in ('Washington D.C', 'Another State')

Maybe? Not tested.

-- Edit

Disregard this; as suggested by the poster above it needs a 'group by' to work.

Answer 2

Can you give this a try?

SELECT
  c.Name AS 'CountryName',
  ISNULL(COUNT(*), 0) as 'ProvinceCount'
FROM 
  Country c LEFT OUTER JOIN Province p ON p.CountryID=c.ID and p.Name <> 'Washington D.C.'
WHERE
  c.ID=@idParameter 
GROUP BY c.Name

Answer 3

You need a GROUP BY clause to get a proper count, and you need an outer join to display '0' values for those countries with no valid provinces.

select
  c.Name as 'country name'
  isnull(count(c.Name), 0) as 'provice count'
from
  Country c
left outer join
  Province p on
  p.CountryID = c.[ID]
where
  c.[ID] = @idParameter
  and p.[Name] not in ('Washington D.C', 'Another State')
group by 
  c.Name

Answer 4

You don't want to list out text, a typo can cause a hard to see bug. You also want to eventually make it so the user can have a page to maintain this themselves. So:

ALTER TABLE Province
ADD IsState bit
GO

UPDATE Province
set IsState = 1
where Name not in ('Washington D.C', 'Another State')
GO

UPDATE Province
SET IsState = 0
WHERE IsState IS NULL
GO

-- double check the data at this point by browsing it...

SELECT  c.name AS 'country name',
isnull(count(1), 0) AS 'provice count'
FROM  Country c
INNER JOIN Province p 
ON  p.CountryID = c.[ID]
WHERE c.[ID] = @idParameter
AND p.IsState = 1
GROUP BY c.name
ORDER BY 1  
GO