数据表在单个连接中连接多个表

Question

我希望在一个连接语句中将多个表链接在一起。即table a 加入table b 加入table c。

# create some tables
a = data.table(id = c(1,2,3,4),amount = c(2,2,3,4))
b = data.table(id=c(1,2,3,4), id2=c(1,1,2,2), multiplier = c(4,5,6,7))
c = data.table(id=c(1,2,3,4))
d = data.table(id2=c(1,2), amount = c(2,20))

接下来我可以将表c 加入a 和b

c[b, on = .(id), `:=` (multiplier = i.multiplier, m2 = multiplier*am)][a, on = .(multiplier==id), am := i.amount]

   id multiplier am m2
1:  1          4  4 16
2:  2          5 NA NA
3:  3          6 NA NA
4:  4          7 NA NA

如何将表b 加入d，然后将c 加入b？

以下内容不起作用：

c[b, on = .(id), `:=` (multiplier = i.multiplier, m2 = multiplier*am)][d, on = .(id2==id2), am := i.amount]

c[b, on = .(id), `:=` (multiplier = i.multiplier, m2 = multiplier*prod(b[d, on = .(id2), amount])]

Answer 1

我搞错了：

c[, m2 := b[, by=id, multiplier*prod(d[.SD, on =.(id2), amount])]$V1]