How to perform multiple left joins using dplyr in R [duplicate]

You can use a nested left_join

 library(dplyr)
 left_join(x, y, by='Flag') %>%
                left_join(., z, by='Flag') 

Or another option would be to place all the datasets in a list and use merge from base R with Reduce

Reduce(function(...) merge(..., by='Flag', all.x=TRUE), list(x,y,z))

Or we have join_all from plyr. Here we place the dataframes in a list and use the argument type="left" for a left join.

library(plyr)
join_all(list(x,y,z), by='Flag', type="left")

As @JBGruber mentioned in the comments, it can also be done via purrr

library(purrr)
library(dplyr)
purrr::reduce(list(x,y,z), dplyr::left_join, by = 'Flag')