SQL Server : function not working in trigger - sql-server

Question

I have trigger on insert like that :
ALTER TRIGGER [dbo].[trTblNameSetRefNo]
ON [dbo].[TblName]
AFTER INSERT
AS BEGIN
UPDATE TblName
SET RefNumber = dbo.GetNextRefNo(i.SomeField)
FROM TblName
INNER JOIN inserted i on i.ID = TblName.ID
END
But it doesn't work. When I 'hardcode' field RefNumber , for example : SET RefNumber = 'test', it works correctly.
Also when I call function outside of the trigger, it return proper result.
And this is my function that must return value in trigger :
ALTER FUNCTION [dbo].[GetNextRefNo]
(#ValueField INT)
RETURNS NVARCHAR(250)
AS
BEGIN
DECLARE #lastId INT;
DECLARE #result NVARCHAR(25);
DECLARE #CurrentIdentifier NVARCHAR(25);
SELECT TOP 1
#lastId = CAST(Substring(RefNumber, Charindex('-', RefNumber) + 4, Len(RefNumber )) AS INT) + 1
FROM
TblName
ORDER BY
ID DESC
IF ##ROWCOUNT < 1 --if empty table , set start number
BEGIN
SET #lastId = 1000
END
SELECT #CurrentIdentifier = 'SIT'
SET #result = #CurrentIdentifier + '-' + Substring ( Cast(Year(Getdate()) AS NVARCHAR), 3, 2) + '-' + Cast(#lastId AS NVARCHAR)
RETURN #result
END
Any ideas what I'm doing wrong?

Answer 1

Your function returns what is probably an incorrect result when RefNumber = '' and it returns NULL when RefNumber = NULL, as would happen for a newly inserted record.
The design of the above code is beyond bad.
Suggested approach:
Use ID field as the counter and generate RefNumber based on that, while concatating Year as needed.
Example function:
ALTER FUNCTION [dbo].[GetNextRefNo] (#ID INT)
returns NVARCHAR(250)
AS
BEGIN
DECLARE #lastId INT;
DECLARE #result NVARCHAR(25);
DECLARE #CurrentIdentifier NVARCHAR(25);
SELECT #CurrentIdentifier = 'SIT'
SET #ID = #ID + 1000
SET #result = #CurrentIdentifier + '-' + Substring ( Cast(Year(Getdate()) AS NVARCHAR), 3, 2) + '-' + Cast(#ID AS NVARCHAR)
RETURN #result
END
Example Trigger:
ALTER TRIGGER [dbo].[trTblNameSetRefNo] ON [dbo].[TblName]
AFTER INSERT AS BEGIN
UPDATE TblName
SET RefNumber = dbo.GetNextRefNo(i.ID)
FROM TblName
INNER JOIN inserted i on i.ID = TblName.ID
END
Alternatively create another table to hold the current RefNumber seed.