分组求和

刚遇到一个问题，就是分组求和在R里怎么实现比较简便？
对应SQL语句为：SELECT customer, sum(consumption) FROM consume group by customer

然后就是类似的，分组计数怎么做？

a=data.frame(customer=c('a','b','a','m','a','b'),consumption=1:6)
a
  customer consumption
1        a           1
2        b           2
3        a           3
4        m           4
5        a           5
6        b           6
tapply(a$consumption,a$customer,sum)
a b m
9 8 4
tapply(a$consumption,a$customer,length)
a b m
3 2 1

　　

在搞复杂点。

a=data.frame(customer=c('a','b','a','m','a','b'),consumption=1:6,groups=c('A','B','A','B','A','B'))

aggregate(a$consumption, list(a$customer,a$groups), sum)
 Group.1 Group.2 x
1       a       A 9
2       b       B 8
3       m       B 4


library(data.table)
consume<-data.table(consume)
setkey(consume,customer)
a1<-consume[,sum(consumption),customer]