Columns to merge on can be specified by name, number or by a logical vector: the name 'row.names' or the number 0 specifies the row names. If specified by name it must correspond uniquely to a named column in the input. In R you use the merge function to combine data frames. This powerful function tries to identify columns or rows that are common between the two different data frames. How to use merge to find the intersection of data The simplest form of merge finds the.
merge is a generic function whose principal method is for data frames: the default method coerces its arguments to data frames and calls the
By default the data frames are merged on the columns with names they both have, but separate specifications of the columns can be given by
by.y. The rows in the two data frames that match on the specified columns are extracted, and joined together. If there is more than one match, all possible matches contribute one row each. For the precise meaning of ‘match’, see
Columns to merge on can be specified by name, number or by a logical vector: the name
'row.names' or the number
0 specifies the row names. If specified by name it must correspond uniquely to a named column in the input.
by or both
by.y are of length 0 (a length zero vector or
NULL), the result,
r, is the Cartesian product of
dim(r) = c(nrow(x)*nrow(y), ncol(x) + ncol(y)).
all.x is true, all the non matching cases of
x are appended to the result as well, with
NA filled in the corresponding columns of
y; analogously for
If the columns in the data frames not used in merging have any common names, these have
'.y' by default) appended to try to make the names of the result unique. If this is not possible, an error is thrown.
by.x column name matches one of
y, and if
no.dups is true (as by default), the y version gets suffixed as well, avoiding duplicate column names in the result.
The complexity of the algorithm used is proportional to the length of the answer.
In SQL database terminology, the default value of
all = FALSE gives a natural join, a special case of an inner join. Specifying
all.x = TRUE gives a left (outer) join,
all.y = TRUE a right (outer) join, and both (
all = TRUE) a (full) outer join. DBMSes do not match
NULL records, equivalent to
incomparables = NA in R.