> plyr::join(data1,data2)
Joining by: id, city, nationality
  id    city nationality Count Area
1  1      GZ          CN   100    3
2  2      ZZ          CN    30    5
3  3      BJ          CN    20   NA
4  4 Newyork          US    40    3
5  5  London          BR    50   NA
> plyr::join(data1,data2, type = 'left', match = 'first')
Joining by: id, city, nationality
  id    city nationality Count Area
1  1      GZ          CN   100    3
2  2      ZZ          CN    30    5
3  3      BJ          CN    20   NA
4  4 Newyork          US    40    3
5  5  London          BR    50   NA
> plyr::join(data1,data2, type = 'inner')
Joining by: id, city, nationality
  id    city nationality Count Area
1  1      GZ          CN   100    3
2  2      ZZ          CN    30    5
3  4 Newyork          US    40    3
> plyr::join(data1,data2, type = 'right')
Joining by: id, city, nationality
  id    city nationality Count Area
1  1      GZ          CN   100    3
2  2      ZZ          CN    30    5
3  3      SH          CN    NA    7
4  4 Newyork          US    40    3
5  5   Paris          Fr    NA    2
> plyr::join(data1,data2, type = 'full',by='city')
  id    city nationality Count Area
1  1      GZ          CN   100    3
2  2      ZZ          CN    30    5
3  3      BJ          CN    20   NA
4  4 Newyork          US    40    3
5  5  London          BR    50   NA
6  3      SH          CN    NA    7
7  5   Paris          Fr    NA    2
> plyr::join(data1,data2, type = 'inner',by='city')
  id    city nationality Count id nationality Area
1  1      GZ          CN   100  1          CN    3
2  2      ZZ          CN    30  2          CN    5
3  4 Newyork          US    40  4          US    3