>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

Assuming that @IssueDate is not null, it should work. Perhaps @IssueDate is also null here. You may try using
set @DatePaidTo = COALESCE(dateadd(month, @MonthsToAdd, @IssueDate), @IssueDate, CURRENT_TIMESTAMP) -- to have current date in case of nulls