SQL code inside SQL Scalar-value function - want to generalize - optimize - sql-server

Question

I got this code, I would like to optimize.
I basically can add new columns to "Disp" table later on, and I don't want to come back modify this function.
I cannot use dynamic SQL. Right? Is there anything else that would work in my case?
This is the function:
ALTER FUNCTION [GetDate]
(#hdrnumber INT, #DateColName VARCHAR(50))
RETURNS DATETIME
AS
BEGIN
DECLARE #dt DATETIME
SELECT #dt = CASE
WHEN #DateColName = 'ord_bookdate' THEN [ord_bookdate]
WHEN #DateColName = 'ord_startdate' THEN [ord_startdate]
WHEN #DateColName = 'ord_completiondate' THEN [ord_completiondate]
WHEN #DateColName = 'pack_date_from' THEN [pack_date_from]
WHEN #DateColName = 'pack_date_to' THEN [pack_date_to]
END
FROM [Disp]
WHERE [hdrnumber] = #hdrnumber
RETURN #dt
END
(removed some of the code, because it's a long one, but hopefully what I left in here will make sense to you guys)
how do i use this function?
well it basically looks like this:
insert into tablename (...)
select somedate, [GetDate](somedate, somecolumn)
from sometable
where 1 = 1

Answer 1

Certainly agree with comments provided in previous two answers.
Anyways, you could write following function to get Column names of a given table and then
compare the column names to #DatecolumName to return the value from it..
Create
function [dbo].[ftTableSchema](#TableName varchar(100)) returns table as
return
--Declare #tableName varchar(30); select #TABLENAME='excelInBom'
SELECT ColumnName=Column_Name
,DataType= case data_type
When 'DECIMAL' then 'DECIMAL('+convert(varchar,Numeric_precision)+','+Convert(varchar,Numeric_scale)+')'
When 'NUMERIC' then 'DECIMAL('+convert(varchar,Numeric_precision)+','+Convert(varchar,Numeric_scale)+')'
when 'VARCHAR' then 'VARCHAR('+Convert(varchar,Character_maximum_length)+')'
WHEN 'CHAR' THEN 'CHAR('+Convert(varchar,Character_maximum_length)+')'
ELSE data_type
end
,ColumnOrder=Ordinal_Position,* FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME=#tableName

Answer 2

You either need to use dynamic sql which you can't do within a user-defined function (so you'd need to remove it out of the function), or like you are doing with a CASE statement (which isn't fully generic).
I think I'd personally go with the CASE approach to start with and consider dynamic sql if proves to give better performance (maybe with a larger number of different possible fields).
You would have some maintenance work to do to keep the CASE up to date, but I can't imagine more fields would be added that often?

Answer 3

I think you have a bigger design issue here. How in the world are you using this? If you need to specify the column as a string (??!) then dynamic SQL will be >>SO<< much faster in many key situations. Something like this:
declare #sql NVARCHAR(MAX);
set #sql = 'SELECT ' + #DateColName + ' FROM disp WHERE hdrnumber = #hdrnumber';
EXEC (#sql, #hdrnumber);
(just watch out that someone cant SQL inject into #DateColName).
But a bigger issue is the design smell that's all over this code. The fact that this function exists worries me about how you could possibly be using it. Take a bigger look, and maybe post another, more detailed question about your design in general and I think you could have even more helpful answers.