I'm missig the CTE/Recursive approaches to solving these issues. While usually not single pass the beat triangular joins

For Running total see: http://shahamishm.blogspot.com/2009/04/how-to-do-running-sum-using-cte-sql.html

For finding previous row ordered by key_nr and trans_date(clustered key):

WITH trans_cte(key_nr, trans_date,field1,field2,row_order)

AS(

SELECT

key_nr

,trans_date

,field1

,field2

,ROW_NUMBER()OVER(PARTITIONBY key_nr ORDERBY trans_date ASC) row_order

FROM [dbo].[trans_stage_table]

)

SELECT BASE.key_nr

,BASE.trans_date

,BASE.field1

,BASE.field2

FROM trans_cte BASE

LEFTJOIN trans_cte LAG ON BASE.key_nr = LAG.key_nr

AND BASE.row_order = LAG.row_order+1

and BASE.field1 = LAG.field1

and BASE.field2 = LAG.field2

where LAG.key_nr isnull