SQL Server FUNCTION overflow int vs bigint - sql

Question

I am using a SQL Server function which returns a bigInt and using this in a trigger to assign a value to a column of type bigint. However when I run the trigger, an overflow exception occurs (Arithmetic overflow error converting expression to data type int.), i.e. it treats it as an int, not a bigint
The function is:
ALTER FUNCTION [dbo].[longIntDateTime] ()
RETURNS bigint
AS
BEGIN
-- Declare the return variable here
DECLARE #ResultVar bigint;
DECLARE #now Datetime;
set #now = getdate();
SET #ResultVar=DATEPART(YYYY,#now)*100000000 + DATEPART(MM,#now)*1000000 + DATEPART(DD,#now)*10000 + DATEPART(HH,#now)*100;
-- DATEPART(HH,#now)*100 + DATEPART(MI,#now);
-- Return the result of the function
RETURN (#ResultVar);
END
The trigger is:
ALTER TRIGGER [dbo].[employeesInsert]
ON [dbo].[employees]
AFTER INSERT
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
-- Insert statements for trigger here
UPDATE employees
SET changeTimeStamp = dbo.longIntDateTime()
FROM inserted INNER JOIN employees On inserted._id = employees._id
END
and the table definition is:
CREATE TABLE [dbo].[employees](
[_id] [int] IDENTITY(1,1) NOT NULL,
[employee_name] [varchar](50) NOT NULL,
[password] [varchar](50) NOT NULL,
[isActive] [int] NOT NULL,
[isDeleted] [int] NOT NULL,
[changeTimeStamp] [bigint] NOT NULL,
CONSTRAINT [PK_employees] PRIMARY KEY CLUSTERED ([_id] ASC)
)
ALTER TABLE [dbo].[employees]
ADD CONSTRAINT [DF_employees_isActive] DEFAULT ((0)) FOR [isActive]
ALTER TABLE [dbo].[employees]
ADD CONSTRAINT [DF_employees_isDeleted] DEFAULT ((0)) FOR [isDeleted]
GO
If I take two '0's off the first yyyy part of the function, the trigger succeeds, however as is, it fails.
Clearly the value produced is less than a big int.
any ideas?
anton

Answer 1

The problem is this line of code:
SET #ResultVar=DATEPART(YYYY,#now)*100000000 + DATEPART(MM,#now)*1000000 + DATEPART(DD,#now)*10000 + DATEPART(HH,#now)*100;
The constants are interpreted as int so the entire calculate is done that way. You can fix this easily by casting the first to bigint:
SET #ResultVar=DATEPART(YYYY,#now)*cast(100000000 as bigint)+ DATEPART(MM,#now)*1000000 + DATEPART(DD,#now)*10000 + DATEPART(HH,#now)*100;

Answer 2

It's because DATEPART returns int.Try to cast to bigint before multiplying
cast (DATEPART(YYYY,#now) as bigint)*100000000