QST: DataFrame.join on key columns
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- [ ] I have asked my usage related question on [StackOverflow](https://stackoverflow.com).

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#### Question about pandas
In pandas 1.2.0, DataFrame.join on key columns doesn't seem to follow the rules of SQL right join and outer join.

In the example below, column `B` in the joining results should've preserved its original values in `pdf2`, following the rules of SQL joins.

I'm wondering whether this is intentional. If yes, it would be great I can get help to understand how it works.

```python
>>> import pandas as pd
>>> pdf1 = pd.DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]})
>>> pdf2 = pd.DataFrame({"B": [11, 12, 13], "C": [14, 15, 16]}, index=[2,3,4])
>>> pdf1.join(pdf2, on='B', lsuffix='_left', how='right')
       A  B_left   B   C
NaN  NaN     NaN   2  14
NaN  NaN     NaN   3  15
0.0  1.0     4.0  13  16
>>> pdf1.join(pdf2, on='B', lsuffix='_left', how='outer')
       A  B_left  B     C
0.0  1.0     4.0  4  16.0
1.0  2.0     5.0  5   NaN
2.0  3.0     6.0  6   NaN
NaN  NaN     NaN  2  14.0
NaN  NaN     NaN  3  15.0
>>> pd.__version__
'1.2.0'
```

To reproduce:
```python
import pandas as pd
pdf1 = pd.DataFrame({"A": [1, 2, 3], "B": [4, 5, 6]})
pdf2 = pd.DataFrame({"B": [11, 12, 13], "C": [14, 15, 16]}, index=[2,3,4])
pdf1.join(pdf2, on='B', lsuffix='_left', how='right')
pdf1.join(pdf2, on='B', lsuffix='_left', how='outer')
```
