>>

SET ANSI_NULLS ON
>>GO
>>SET QUOTED_IDENTIFIER ON
>>GO
>>-- =============================================
>>-- Author:		<Author,,Name>
>>-- Create date: <Create Date, ,>
>>-- Description:	<Description, ,>
>>-- =============================================
>>CREATE FUNCTION GetDatePaidTo
>>(
>>	-- Add the parameters for the function here
>>	@PolicyHeaderPK uniqueidentifier
>>)
>>RETURNS datetime
>>AS
>>BEGIN
>>	-- Declare the return variable here
>>	DECLARE @DatePaidTo datetime
>>
>>	-- Add the T-SQL statements to compute the return value here
>>	declare @MonthsToAdd int
>>	declare @PremiumMode varchar(15)
>>	declare @IssueDate datetime
>>	
>>	SELECT @MonthsToAdd = SUM(
>>		CASE [PremiumMode]  -- Add table alias here
>>			 WHEN 'Monthly' THEN NumberOfPremiums * 1 -- add table alias for NumberOfPremiums
>>			 WHEN 'Quarterly' THEN NumberOfPremiums * 3
>>			 WHEN 'Semi-Annually' THEN NumberOfPremiums * 6
>>			 WHEN 'Annually' THEN NumberOfPremiums * 12
>>			 ELSE 0
>>		END )
>>	FROM [dbo].[ReceiptDetails]
>>	inner join [dbo].[Receipts] on ReceiptFK = ReceiptPK  -- looks like this join is not needed, also, add aliases to tables
>>	WHERE 
>>		([PolicyHeaderFK] = @PolicyHeaderPK
>>		AND ReceiptTypeFK = 3)
>>
>>	SELECT @IssueDate = IssueDate -- add alias here
>>	FROM [dbo].[Policyheaders]
>>		INNER JOIN [dbo].[Policies]	ON [PolicyHeaderPK] = [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
>>		inner join [dbo].[Plans] on [PlanFK] = [PlanPK] -- do we need all these joins?
>>		inner join [dbo].[Clients] on [ClientFK] = [ClientPK]
>>		INNER JOIN [dbo].[Status] on [StatusFK] = [StatusPK]
>>	WHERE 
>>		([PolicyHeaderPK] = @PolicyHeaderPK)
>>
>>	SET @DatePaidTo = DATEADD(M, @MonthsToAdd, @IssueDate)
>>	
>>	-- Return the result of the function
>>	RETURN @DatePaidTo
>>
>>END
>>GO

>>
>>Thanks
>
>I put some comments. It may be OK. Keep in mind that scalar functions in T-SQL may be slow.
>Also, I suggest to add table alias for each field, as if someone needs to maintain this code, it's not clear which table which field belongs to.

OK, thanks. All the fields being pulled out/used in the first SELECT are from the ReceiptDetails, but I'll make that clearer in the code.