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merge(x = df1, y = df2, by = "CustomerId", all = TRUE)
merge(x = df1, y = df2, by = "CustomerId", all.x = TRUE)
merge(x = df1, y = df2, by = "CustomerId", all.y = TRUE)
merge(x = df1, y = df2, by = NULL)

merge(x = t1,y=county.fips1, by.x=c('county_name','state_code'),by.y=c('county_name','state_code1'),all.x=TRUE)