sum_up prints detailed summary statistics (corresponds
to Stata summarize)
tab prints distinct rows with their count. Compared to
the dplyr function count, this command adds frequency,
percent, and cumulative percent.
join is a wrapper for dplyr merge functionalities, with
two added functions
The option check checks there are no duplicates in
the master or using data.tables (as in Stata).
The option gen specifies the name of a new variable
that identifies non matched and matched rows (as in Stata).
The option update allows to update missing values of
the master dataset by the value in the using dataset