Say I have a data frame, `df1`, with many columns, and I'm interested in only one column. something like this:

```
               outcome   Feature1   Feature2
Person1          Yes        10         11
Person10         Yes        28.3       9
Person11         Yes        1          0
Person12         No         100        23
Pt33             Yes        34         67
Pt40x            No         42         98
pt80xx           No         0          11

```

And I have another data frame, `df2`, with only some of `df1` and it does not have the `outcome` column. Something like this:

```
                Feature1   Feature2
Person1           10         11
Person10          28.3       9
Pt33              34         67
pt80xx            0          11

```

I want to add the `outcome` column from `df1` to `df2`, only for the existing rows in `df2`, so the final result:

```
              Outcome  Feature1   Feature 2
Person1         Yes      10         11
Person10        Yes      28.3       9
Pt33            Yes      34         67
pt80xx          No       0          11

```