data.frame(product_name = c("A", "B", "B", NA), product_code = c(NA, 1, 2, 1))
# product_name product_code
#            A           NA
#            B            1
#            B            2
#         <NA>            1

data.frame(product_name = c(rep(LETTERS, 2), rep(NA, 48)),
           product_code = c(rep(NA, 10), sample(1:80, 90, replace = TRUE)))