Merge several reference database by common fields.
refdb_merge(..., keep = "fields_all")
reference databases (tibbles).
determines which columns to keep.
a merged reference database (tibble).
Columns are merged only if they are associated to the same field.
keep argument determines which columns are returned as follow.
"fields_all" (the default) returns all the fields
existing in all the reference databases.
"fields_shared" returns only the fields shared by
all the reference databases.
"all" returns all the columns of all the databases.
Columns which are not associated to a field are not merged and are prefixed
with the name of the object they originated from.