An alternative solution...

;WITH cteJune ([level],[SthdiCode],[FunctionalArea],[Qty])

AS

(

SELECT '01','G22','Area1',5 UNION ALL

SELECT '01','G22','Area2',5 UNION ALL

SELECT '01','G22','Area3',5 UNION ALL

SELECT '02','G22','Area1',5 UNION ALL

SELECT '02','G22','Area2',5 UNION ALL

SELECT '02','G22','Area3',20 UNION ALL

SELECT '02','WWW','Area3',5

),

cteJuly ([level],[SthdiCode],[FunctionalArea],[Qty])

AS

(

SELECT '01','G22','Area1',15 UNION ALL

SELECT '01','G22','Area2',15 UNION ALL

SELECT '01','G22','Area3',15 UNION ALL

SELECT '02','G22','Area1',15 UNION ALL

SELECT '02','G22','Area2',15 UNION ALL

SELECT '02','G22','Area3',15 UNION ALL

SELECT '02','WWW','Area3',5

)

SELECT

r.[level]

,r.SthdiCode

,r.FunctionalArea

,r.QtyJune

,r.QtyJuly

,SUM(r.QtyJuly-r.QtyJune) OVER (PARTITION BY r.[level],r.SthdiCode,r.FunctionalArea) AS [Difference]

FROM

(

SELECT DISTINCT

cteJune.[level]

,cteJune.SthdiCode

,cteJune.FunctionalArea

,cteJune.Qty AS QtyJune

,cteJuly.Qty AS QtyJuly

FROM

cteJune

INNER JOIN

cteJuly

ON cteJune.[level] = cteJuly.[level]

AND cteJune.SthdiCode = cteJuly.SthdiCode

AND cteJune.FunctionalArea = cteJuly.FunctionalArea

) r

levelSthdiCodeFunctionalAreaQtyJuneQtyJulyDifference

01G22Area151510

01G22Area251510

01G22Area351510

02G22Area151510

02G22Area251510

02G22Area32015-5

02WWWArea3550