Multiple errors in SQL Server Function - sql

Question

I'm trying to make a function that evaluates the expiration date in a policy and based on that when is called in a Query it returns the text values ('Monthly'..and so on). but i'm getting a few errors of type "Incorrect syntax near Begin, declare expecting (.", "Must declare the scalar variable #policybillid" and in the end of the function "Incorrect syntax near ')'.". Any help would be appreciated.
USE [defaultDB]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE OR ALTER FUNCTION [dbo].[fn_Payment_GetPaymentCoutesByPaymentID]( #policybillid BIGINT, #companyID BIGINT) Returns TABLE
AS
RETURN (
BEGIN( ***Incorrect syntax near 'BEGIN'. Expecting '(', SELECT, or WITH.***
declare #expiresDates as table(expiredOn date) ***Incorrect syntax near 'declare'.*** Expecting '(', or SELECT.
insert into #expiresDates
select top(2) expiredOn from PolicyPaymentPlan
left join PolicyPaymentPlanFee on PolicyPaymentPlan.paymentPlanID= PolicyPaymentPlanFee.paymentPlanID
where PolicyPaymentPlan.policyBillID = #policybillid --Must declare the scalar variable "#policybillid".
order by cuoteID
select * from #expiresDates
declare #diffMonth int
declare #quota varchar(max) = ''
if((select count(*) from #expiresDates) > 1)
BEGIN
set #diffMonth = DATEDIFF(month, (select top(1) expiredOn from #expiresDates), (select top(1) expiredOn from #expiresDates order by expiredOn desc));
set #quota = ( SELECT
CASE
WHEN #diffMonth =1
then
'Monthly'
WHEN #diffMonth =2 THEN
'bimonthly'
WHEN #diffMonth =3 THEN
'trimonthly '
WHEN #diffMonth =4 THEN
'Four-Monthly'
WHEN #diffMonth =6 THEN
'biannual'
ELSE 'Cash'
END
)
END
) ***Incorrect syntax near ')'.***

Answer 1

There are numerous issues and syntax errors on your existing code. The main one is that an inline Table Valued Function can only have a single SELECT statement and nothing else.
So here is what it should look like.
CREATE OR ALTER FUNCTION [dbo].[fn_Payment_GetPaymentCoutesByPaymentID](
#policybillid BIGINT,
#companyID BIGINT
)
RETURNS TABLE
AS RETURN
SELECT Quota =
CASE WHEN COUNT(*) > 1 THEN
CASE DATEDIFF(month, MIN(expiredOn), MAX(expiredOn))
WHEN 1 THEN 'Monthly'
WHEN 2 THEN 'bimonthly'
WHEN 3 THEN 'trimonthly '
WHEN 4 THEN 'Four-Monthly'
WHEN 6 THEN 'biannual'
ELSE 'Cash'
END
END
FROM (
select top (2) expiredOn
from PolicyPaymentPlan pp
join PolicyPaymentPlanFee ppf on pp.paymentPlanID = ppf.paymentPlanID
where pp.policyBillID = #policybillid
order by cuoteID
) pp
;