SQL Creating a function to get the sum based on column name - sql

Question

Im trying to write a simple function that will allow me to get the sum based off the value of a column.
CREATE FUNCTION [GetSumOfColumnByCase](#column varchar(50), #case int)
RETURNS INT
AS
BEGIN
declare #return int
set #return = SUM(CASE WHEN #column = #case THEN 1 ELSE 0 END)
-- Return the result of the function
return #return
END
GO
I call this function like this:
SELECT HouseDescription,
[dbo].[GetSumOfColumnByCase]([HouseTypeId], 1) AS "houseType1",
[dbo].[GetSumOfColumnByCase]([HouseTypeId], 2) AS "houseType2"
Doing things this way forces me to GROUP BY both the houseDescription and the HouseTypeId columns but i just want to GROUP BY the housedescription.
If i do things this way:
SELECT HouseDescription,
SUM(CASE WHEN HouseTypeId = 1 THEN 1 ELSE 0 END) AS "houseType1",
SUM(CASE WHEN HouseTypeId = 2 THEN 1 ELSE 0 END) AS "houseType2"
Its fine, it doesnt force me to GROUP BY HouseTypeId.
Can anyone explain why this is?

Answer 1

When you are using a GROUP BY clause, every column needs to either be in the GROUP BY, or it needs to be aggregated.
In your second example, you are fulfilling these requirements - by placing the SUM around the function call. In your first example, since the function call itself isn't wrapped in an aggregation (SUM, MAX, MIN, etc.), you must place it in the GROUP BY clause in order to not trigger an error.
https://msdn.microsoft.com/en-us/library/ms177673.aspx
I agree with Gordon though, you may want to rethink your strategy for this.

Answer 2

You cannot do what you want with a function, because SQL Server does not support dynamic SQL (readily) in functions. And to handle any column, you would need dynamic SQL.
But, you don't need that anyway. If you want the sum on each row of the original data, you want window functions:
SELECT SUM(CASE WHEN HouseTypeId = 1 THEN 1 ELSE 0 END) OVER () AS houseType1,
SUM(CASE WHEN HouseTypeId = 2 THEN 1 ELSE 0 END) OVER () AS houseType2
. . .
Aggregation is not needed for this query.