Join and overwrite data in one table with data from another table

Question

How to join and overwrite data appears to be a common request, but I have yet to find an elegant solution that applies to an entire dataset.

(Note: to simplify the d

Answer 1

I think it's easiest to go to long form:

md1 = melt(d2, id="id")
md2 = melt(d2, id="id")

Then you can stack them and take the latest value:

res1 = unique(rbind(md1, md2), by=c("id", "variable"), fromLast=TRUE)

I'd also like to know how this can be done if you only want to update the NA values in [d3], that is, make sure existing non-NA values are not overwritten.

You can exclude rows from the update table, md2, if they appear in md3:

md3 = melt(d3, id="id")

res3 = unique(rbind(md3, md2[!md3, on=.(id, variable)]), 
  by=c("id", "variable"), fromLast=TRUE)   

dcast can be used to go back to wide format if necessary, e.g., dcast(res3, id ~ ...).