弱问:现有data a数据如下ID,name,sex,station,expense,date
002,suzuki,F,shimokitazawa,13650,2013/10/5
005,yoshiko,F,tokyo,5840,2013/1/26
005,yoshiko,F,shibuya,8640,2013/5/3
data b 数据如下
ID,name,sex,station,expense,date 
001,fujita,M,iidabashi,6160,2013/4/2
001,fujita,M,tokyo,5840,2013/12/20
003,takahashi,M, funabashi,17800,2013/8/21
003,takahashi,M, shimjuku,10800,2013/11/1
004,tanaka,M,yokohama,15830,2013/2/25
合并后,想使用「If Last, If first,retain」
对相同ID的expense进行求和增加一个变数。如何实现。万分感谢