如何加入（合并）数据框（内部，外部，左侧，右侧）？

Given two data frames:

给出两个数据框:

df1 = data.frame(CustomerId = c(1:6), Product = c(rep("Toaster", 3), rep("Radio", 3)))df2 = data.frame(CustomerId = c(2, 4, 6), State = c(rep("Alabama", 2), rep("Ohio", 1)))df1#  CustomerId Product#           1 Toaster#           2 Toaster#           3 Toaster#           4   Radio#           5   Radio#           6   Radiodf2#  CustomerId   State#           2 Alabama#           4 Alabama#           6    Ohiodf1 = data.