What makes a Scalar Valued Function "deterministic"

Question

Hi all,
I have the following column definition in a CREATE TABLE statement:
[Batch] AS [ÜNB] + '-' + [TypeOfData] + ' {' + CONVERT(NCHAR(4), [Validity Year])
+ '} {' + CAST([Actuality Year] AS CHAR(4)) + '-'
+ IIF([Actuality Month] < 10, '0', '')
+ CAST([Actuality Month] AS VARCHAR(2)) + '}' PERSISTED
It is accepted as deterministic by SQL Engine. When coding the same formula as a function, SQL Engine marks the function as non-deterministic.
CREATE FUNCTION [dbo].[pamDataflowBatchName]
@ÜNB VARCHAR(10), @TypeOfData VARCHAR(20) = 'Transactions', @ValidityYear SMALLINT, @ActualityYear SMALLINT = 2014, @ActualityMonth TINYINT = 5
RETURNS NVARCHAR(80)
WITH RETURNS NULL ON NULL INPUT
AS
BEGIN
DECLARE @ValidityYearOK SMALLINT = IIF(@ValidityYear <= @ActualityYear, @ValidityYear, @ActualityYear);
DECLARE @ActualityMonthOK TINYINT = IIF(@ActualityMonth > 0 AND @ActualityMonth <= 12, @ActualityMonth, 5);
RETURN @ÜNB + '-' + @TypeOfData + ' {' + CONVERT(NCHAR(4), @ValidityYearOK) + '} {' + CAST(@ActualityYear AS CHAR(4))
+ '-' + IIF(@ActualityMonthOK < 10, '0', '') + CAST(@ActualityMonthOK AS VARCHAR(2)) + '}'
END
What rules must I obey to make that function 'deterministic'?
Regards Uwe

Answer 1

Hi Erland,
I fully agree! The problem here is, that the SSDT-deploy automates the process intransparently and there is no chance to intervene. So the solution is casting in the CREATE TABLE which is not the right place to limit the result size. The CLR-aggregate
class has implemented this correctly using an attribute "MaxByteSize=nnnn" in the C# code so the one person gets the limit and deploy takes it and lets SQL check whether this limit is usable or not in the SQL context.
So my workaround is:
CREATE TABLE [dbo].[TestCLRFunctions]
([Computed Column One] AS CAST(dbo.[MyCLR-UDF]('A', 'B', 2010, 2014, 12) AS VARCHAR(160)) PERSISTED NOT NULL
CONSTRAINT [PK_TestCLRFunctions] PRIMARY KEY ([Computed Column One]))
Easy but bad programming practice, because two parties with different scopes have to cooperate to implement one little task. This is surely error-prone like hell. (The last sentence contains some HTML artifacts and appears blue here. No emphasizing
Regards Jörg