how to use merge() to update a table in R

Question

I\'m trying to figure out how to use merge() to update a database.

Here is an example. Take for example the data frame foo

Answer 1

I would also like to present an sql-solution using library sqldf and the R integrated sqlite-database. I like the simplicity, accuratness and power of sql.
Accurateness: since I can exactly define which object=rows I want to change without considering the order of a data.frame (foo.id = bar.id).
Power: in WHERE after SET and WHERE (third row) I can define all conditions I want to consider to update.
Simplicity: the syntax is more readable than using index in vectors, matrix or dataframes.

library(sqldf)

# I changed index to id since index does not work. 
#   Obviously index is a key word in sqlite.

(foo <- data.frame(id=c('a', 'b', 'c', 'd'), value=c(100, 101, NA, NA)))
(bar <- data.frame(id=c('c', 'd'), value=c(200, 201)))

sqldf(c(paste("UPDATE foo"
             ," SET value = (SELECT bar.value FROM bar WHERE foo.id = bar.id)"
             ," WHERE value IS NULL"
             )
        , " SELECT * FROM main.foo"
    )
)

Which gives

  id value
1  a   100
2  b   101
3  c   200
4  d   201

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