SQL Conditional join inside a function - sql

Question

This question has been asked many times on SO but I never quite found the answer to it, they are mostly solutions to avoid the problem altogether.
I'm working with SQL MS and I'm trying to build a query inside a function (for security reasons) that will either return a table or it's unnested version by country.
meaning that the function should either be
SELECT * FROM SALES AS S
or
SELECT
S.*,
C.Country,
C.CountryPercentage * S.AmountWithouthVAT as CountryValue
FROM SALES AS S
INNER JOIN CountryAllocation AS C ON S.CountryAllocationID = C.CountryAllocationID
(the fact that this join will make a single row into many rows is why I don't simply use the above one. And the reason why I don't make the join outside the function is because the person running the function will not have access to either of the tables. Also note that because of the way permissions in SQL Server work a dynamic query will require permission evaluation, meaning that is not a feasible option unless I'm to develop a structure around certificates)
So, now I got 2 problems:
The output table might or might not have the columns Country and CountryValue causing problems when defining the output type of the function
The actual way to have a function parameter to switch between the 2 versions of the table.
I've got a solution, but this code pains my eyes to look upon:
CREATE FUNCTION [dbo].[fn_I_view] (#Type int)
RETURNS #OutTable TABLE
(
SaleID int,
AmountWithouthVAT decimal(18, 2),
Country varchar(50),
AlocationPercentage decimal(18, 2)
)
AS
BEGIN
WITH
Out1 AS
(
SELECT
S.*,
NULL as Country,
NULL as AlocationPercentage
FROM Sales AS S
WHERE #Type = 1
),
Out2 AS
(
SELECT
S.*,
C.Country,
C.CountryPercentage * S.AmountWithouthVAT as CountryValue
FROM SALES AS S
INNER JOIN CountryAllocation AS C ON S.CountryAllocationID = C.CountryAllocationID
WHERE #Type = 2
)
INSERT INTO #OutTable
SELECT * FROM Out1
UNION ALL
SELECT * FROM Out2
RETURN
END
GO
so, I can't exactly fix the first problem, only worked around it by making SELECT * from [INV].[fn_I_ViewAllMyInvoices](1) still return those 2 extra columns with NULL and I didn't fix the second problem either, as I'm calculating both queries when I only needed 1 of them (and as you can expect this is a demo code, the real deal is way more complex)
Is there any way to improve this code?/solve the problem in a different way? performance, readability as well as maintenance improvements are all welcome

Answer 1

You don't need to calculate both. Just do:
BEGIN
IF #type = 1
BEGIN
INSERT INTO #OutTable
SELECT S.*, NULL as Country, NULL as AlocationPercentage
FROM Sales s;
END;
ELSE
BEGIN
INSERT INTO #OutTable
SELECT S.*, C.Country, C.CountryPercentage * S.AmountWithouthVAT as CountryValue
FROM SALES S JOIN
CountryAllocation C
ON S.CountryAllocationID = C.CountryAllocationID;
END;
RETURN;
END;