Hi,

I have the following function is SQL Server 2008:

USE [SIAS]
GO
/****** Object:  UserDefinedFunction [dbo].[GetNextPremiumDue]    Script Date: 02/21/2011 09:42:08 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:		<Author,,Name>
-- Create date: <Create Date, ,>
-- Description:	<Description, ,>
-- =============================================
CREATE FUNCTION [dbo].[GetNextPremiumDue]
(
	-- Add the parameters for the function here
	@PolicyHeaderPK uniqueidentifier
)
RETURNS datetime
AS
BEGIN
	-- Declare the return variable here
	DECLARE @PremiumDue datetime
	
	-- Add the T-SQL statements to compute the return value here
	DECLARE @DatePaidTo datetime
	declare @MonthsToAdd int
	declare @PremiumMonths int
	declare @IssueDate datetime
	
	SELECT @MonthsToAdd = SUM(
		CASE [PremiumMode] 
			 WHEN 'Monthly' THEN [ReceiptDetails].[NumberOfPremiums] * 1
			 WHEN 'Quarterly' THEN [ReceiptDetails].[NumberOfPremiums] * 3
			 WHEN 'Semi-Annually' THEN [ReceiptDetails].[NumberOfPremiums] * 6
			 WHEN 'Annually' THEN [ReceiptDetails].[NumberOfPremiums] * 12
			 ELSE 0
		END )
	FROM [dbo].[ReceiptDetails]
	inner join [dbo].[Receipts] on [ReceiptDetails].[ReceiptFK] = [Receipts].[ReceiptPK]
	WHERE 
		([Receipts].[PolicyHeaderFK] = @PolicyHeaderPK
		AND ReceiptTypeFK = 3)

	SELECT @IssueDate = [Policies].[IssueDate],
			@PremiumMonths = 
		CASE [Policies].[PremiumMode] 
			 WHEN 'Monthly' THEN 1
			 WHEN 'Quarterly' THEN 3
			 WHEN 'Semi-Annually' THEN 6
			 WHEN 'Annually' THEN 12
			 ELSE 0
		END 
	FROM [dbo].[Policyheaders]
		INNER JOIN [dbo].[Policies]	ON [Policyheaders].[PolicyHeaderPK] = [Policies].[PolicyHeaderFK]
		INNER JOIN (SELECT [PolicyHeaderFK], MAX(LastChanged) as LastChanged
						FROM [dbo].[Policies]
						GROUP BY [PolicyHeaderFK]) subquery ON [Policies].[PolicyHeaderFk] = subquery.policyHeaderFK AND Policies.LastChanged = subquery.LastChanged
	WHERE 
		([Policyheaders].[PolicyHeaderPK] = @PolicyHeaderPK)

	SET @DatePaidTo = ISNULL(DATEADD(M, @MonthsToAdd, @IssueDate), @IssueDate)
	SET @PremiumDue = DATEADD(M, @PremiumMonths, @DatePaidTo)
	
	-- Return the result of the function
	RETURN @PremiumDue

END

The problem I am having us that NULL is being returned in the situation where there are no records in the ReceiptDetails table. In this case I would like to set the @DatePaidTo to the @IssueDate, but apparently

SET @DatePaidTo = ISNULL(DATEADD(M, @MonthsToAdd, @IssueDate), @IssueDate)

Isn't doing what I thought it would do. Or maybe I'm missing some other logic problem here