Issue with While Loop in SQL function - sql

Question

I'm writing a function that should add each line item quantity multiplied by its unit cost and then iterating through the entire pickticket (PT). I don't get an error when altering the function in SQL Server or when running it, but it gives me a 0 as the output each time.
Here is an example:
[PT 1]
[Line 1 - QTY: 10 Unit Cost: $5.00] total should be = $50.00
[Line 2 - QTY: 5 Unit Cost: $2.50] total should be = $12.50
The function should output - $62.50
Not really sure what I'm missing here, but would appreciate the help.
Alter Function fn_CalculateAllocatedPTPrice
(#psPickTicket TPickTicketNo)
-------------------------------
Returns TInteger
As
Begin
Declare
#iReturn TInteger,
#iTotalLineNumbers TInteger,
#iIndex TInteger,
#fTotalCost TFloat;
set #iIndex = 1;
set #iTotalLineNumbers = (ISNULL((select top 1 PickLineNo
from tblPickTicketDtl
where PickTicketNo = #psPickTicket
order by PickLineNo desc), 0)) /* This returns the highest line number */
while(#iIndex <= #iTotalLineNumbers)
BEGIN
/* This should be adding up the total cost of each line item on the PT */
set #fTotalCost = #fTotalCost + (ISNULL((select SUM(P.RetailUnitPrice*P.UnitsOrdered)
from tblPickTicketDtl P
left outer join tblCase C on (P.PickTicketNo = C.PickTicketNo)
where P.PickTicketNo = #psPickTcket
and P.PickLineNo = #iIndex
and C.CaseStatus in ('A','G','K','E','L','S')), 0))
set #iIndex = #iIndex + 1;
END
set #iReturn = #fTotalCost;
_Return:
Return(#iReturn);
End /* fn_CalculateAllocatedPTPrice */

Answer 1

It seems simple aggregation should suffice
A few points to note:
WHILE loops and cursors are very rarely needed in SQL. You should stick to set-based solutions, and if you find yourself writing a loop you shuold question your code from its beginnings.
Scalar functions are slow and inefficient. Use an inline Table function, which you can correlate with your main query either with an APPLY or a subquery
Your left join becomes an inner join because of the where predicate
User defined types are not normally a good idea (when they are just aliasing system types)
CREATE OR ALTER FUNCTION fn_CalculateAllocatedPTPrice
(#psPickTicket TPickTicketNo)
RETURNS TABLE AS RETURN
SELECT fTotalCost = ISNULL((
SELECT SUM(P.RetailUnitPrice * P.UnitsOrdered)
from tblPickTicketDtl P
join tblCase C on (P.PickTicketNo = C.PickTicketNo)
where P.PickTicketNo = #psPickTcket
and C.CaseStatus in ('A','G','K','E','L','S')
), 0);
GO