You can use the following basic syntax to merge two data frames in R based on their rownames:

#inner join
merge(df1, df2, by=0)

#left join
merge(df1, df2, by=0, all.x=TRUE)

#outer join
merge(df1, df2, by=0, all=TRUE)

By using the argument by=0, we’re able to tell R that we want to merge using the rownames of the data frames.

The following examples show how to use each method with the following two data frames:

#create first data frame
df1 <- data.frame(points=c(99, 90, 86, 88, 95),
                  assists=c(33, 28, 31, 39, 34))

rownames(df1) <- c(1, 2, 3, 4, 5)

df1

  points assists
1     99      33
2     90      28
3     86      31
4     88      39
5     95      34

#create second data frame
df2 <- data.frame(rebounds=c(17, 15, 22, 26, 25),
                  blocks=c(7, 7, 15, 12, 14))

rownames(df2) <- c(3, 4, 5, 6, 7)

df2

  rebounds blocks
3       17      7
4       15      7
5       22     15
6       26     12
7       25     14

Example 1: Perform Inner Join Using Row Names

The following code shows how to perform an inner join on two data frames using the row names:

#perform inner join using row names
merge(df1, df2, by=0)

  Row.names points assists rebounds blocks
1         3     86      31       17      7
2         4     88      39       15      7
3         5     95      34       22     15

Notice that only the rows whose row names belong in both data frames are kept in the final merged data frame. 

Example 2: Perform Left Join Using Row Names

The following code shows how to perform a left join on two data frames using the row names:

#perform left join using row names
merge(df1, df2, by=0, all.x=TRUE)

  Row.names points assists rebounds blocks
1         1     99      33       NA     NA
2         2     90      28       NA     NA
3         3     86      31       17      7
4         4     88      39       15      7
5         5     95      34       22     15

Notice that all of the rows from the first data frame are kept in the final merged data frame.

Example 3: Perform Outer Join Using Row Names

The following code shows how to perform an outer join on two data frames using the row names:

#perform outer join using row names
merge(df1, df2, by=0, all=TRUE)

  Row.names points assists rebounds blocks
1         1     99      33       NA     NA
2         2     90      28       NA     NA
3         3     86      31       17      7
4         4     88      39       15      7
5         5     95      34       22     15
6         6     NA      NA       26     12
7         7     NA      NA       25     14

Notice that all of the rows from both data frames are kept in the final merged data frame.

Additional Resources

The following tutorials explain how to perform other common functions related to data frames in R: