Restrict SQL subquery in SELECT - sql

Question

I thought the subquery within the select statement will be restricted by the FROM and/or JOIN statements.
Therefore, my query always returns an error because there is more than one row in the subquery.
SELECT
dbo.Countries.Name,
dbo.Countries.ISO2,
(SELECT dbo.CountryFields.Field
FROM dbo.CountryFields
WHERE dbo.CountryFields.Field = 'Population') AS Population
FROM
dbo.CountryFields
INNER JOIN
dbo.Countries ON (dbo.CountryFields.Countries_Id = dbo.Countries.Countries_Id)
How can I restrict the number of rows in my subquery?
Do I need there also an inner join Statement inside the subquery? I hoped the subquery will inherit from normal SELECT so I don't need manual restrictions.
The column "Field" contains more than "Population" and I would like to show more rows in the SELECT statement with subselects but now ... I can't even get one column to work. :-(

Answer 1

I think you want something like this:
SELECT
a.Name,
a.ISO2,
(SELECT TOP 1 b.Field FROM dbo.CountryFields b WHERE b.Countries_Id = a.Countries_Id AND b.Field = 'Population') AS Population,
(SELECT TOP 1 b.Field FROM dbo.CountryFields b WHERE b.Countries_Id = a.Countries_Id AND b.Field = 'Capital') AS Capital,
(SELECT TOP 1 b.Field FROM dbo.CountryFields b WHERE b.Countries_Id = a.Countries_Id AND b.Field = 'Area') AS Area
FROM
dbo.Countries a
Of course there are ways to optimize the above query, but it's always a tradeoff between readability and speed.
Good luck!

Answer 2

I think that something like this is the proper query:
SELECT
C.Name,
C.ISO2,
ISNULL(CF_POP.Value,0) AS [Population],
ISNULL(CF_F2.Value,0) AS [Field2],
ISNULL(CF_F3.Value,0) AS [Field3]
FROM
dbo.Countries AS C
LEFT JOIN dbo.CountryFields AS CF_POP ON (C.Countries_Id = CF_POP.Countries_Id) AND (CF_POP.Field = 'Population')
LEFT JOIN dbo.CountryFields AS CF_F2 ON (C.Countries_Id = CF_F2.Countries_Id) AND (CF_F2.Field = 'Field2')
LEFT JOIN dbo.CountryFields AS CF_F3 ON (C.Countries_Id = CF_F3.Countries_Id) AND (CF_F3.Field = 'Field3')
In this example you connect each row from CountryFields as a column. I use LEFT JOIN, because I don't know how complete is your data (if you want to see blanks you have to remove ISNULL). I also put column Value, because I suppose that there must be second column which corresponds to CountryFields.Field. This also can be done with CROSS APPLY, but in that case syntax will be different.