Summing From Consecutive Rows - tsql

Question

Assume we have a table and we want to do a sum of the Expend column so that the summation only adds up values of the same Week_Name.
SN Week_Name Exp Sum
-- --------- --- ---
1 Week 1 10 0
2 Week 1 20 0
3 Week 1 30 60
4 Week 2 40 0
5 Week 2 50 90
6 Week 3 10 0
I will assume we will need to `Order By' Week_Name, then compare the previous Week_Name(previous row) with the current row Week_name(Current row).
If both are the same, put zero in the SUM column.
If not the same, add all expenditure, where Week_Name = Week_Name(Previous row) and place in the Sum column. The final output should look like the table above.
Any help on how to achieve this in T-SQL is highly appreciated.

Answer 1

Okay, I was eventually able to resolve this issue, praise Jesus! If you want the exact table I gave above, you can use GilM's response below, it is perfect. If you want your table to have running Cumulatives, i.e. Rows 3 shoud have 60, Row 5, should have 150, Row 6 160 etc. Then, you can use my code below:
USE CAPdb
IF OBJECT_ID ('dbo.[tablebp]') IS NOT NULL
DROP TABLE [tablebp]
GO
CREATE TABLE [tablebp] (
tablebpcCol1 int PRIMARY KEY
,tabledatekey datetime
,tableweekname varchar(50)
,expenditure1 numeric
,expenditure_Cummulative numeric
)
INSERT INTO [tablebp](tablebpcCol1,tabledatekey,tableweekname,expenditure1,expenditure_Cummulative)
SELECT b.s_tablekey,d.PK_Date,d.Week_Name,
SUM(b.s_expenditure1) AS s_expenditure1,
SUM(b.s_expenditure1) + COALESCE((SELECT SUM(s_expenditure1)
FROM source_table bs JOIN dbo.Time dd ON bs.[DATE Key] = dd.[PK_Date]
WHERE dd.PK_Date < d.PK_Date),0)
FROM source_table b
INNER JOIN dbo.Time d ON b.[Date key] = d.PK_Date
GROUP BY d.[PK_Date],d.Week_Name,b.s_tablekey,b.s_expenditure1
ORDER BY d.[PK_Date]
;WITH CTE AS (
SELECT tableweekname
,Max(expenditure_Cummulative) AS Week_expenditure_Cummulative
,MAX(tablebpcCol1) AS MaxSN
FROM [tablebp]
GROUP BY tableweekname
)
SELECT [tablebp].*
,CASE WHEN [tablebp].tablebpcCol1 = CTE.MaxSN THEN Week_expenditure_Cummulative
ELSE 0 END AS [RunWeeklySum]
FROM [tablebp]
JOIN CTE on CTE.tableweekname = [tablebp].tableweekname

Answer 2

I'm not sure why your SN=6 line is 0 rather than 10. Do you really not want the sum for the last Week? If having the last week total is okay, then you might want something like:
;WITH CTE AS (
SELECT Week_Name,SUM([Expend.]) as SumExpend
,MAX(SN) AS MaxSN
FROM T
GROUP BY Week_Name
)
SELECT T.*,CASE WHEN T.SN = CTE.MaxSN THEN SumExpend
ELSE 0 END AS [Sum]
FROM T
JOIN CTE on CTE.Week_Name = T.Week_Name
Based on the requst in the comment wanting a running total in SUM you could try this:
;WITH CTE AS (
SELECT Week_Name, MAX(SN) AS MaxSN
FROM T
GROUP BY Week_Name
)
SELECT T.SN, T.Week_Name,T.Exp,
CASE WHEN T.SN = CTE.MaxSN THEN
(SELECT SUM(EXP) FROM T T2
WHERE T2.SN <= T.SN) ELSE 0 END AS [SUM]
FROM T
JOIN CTE ON CTE.Week_Name = T.Week_Name
ORDER BY SN