Combine 2 dataframes share the same column in pandas

Question

I want to combine two dataframes into one

df1

 key  columnA
  1    0
  1    1
  1    2

df2

 key columnB
  1    3
  1    5
  1    7

result

 key columnA columnB
  1    0       3
  1    1       5
  1    2       7

resulting dataframe's column ordering is not important

Edit: I have tried

 pd.merge(df1, df2, on ='key', how = 'inner')

it gives me a df of

Possible duplicate of [Pandas Combining 2 Data Frames (join on a common column)](https://stackoverflow.com/questions/18792918/pandas-combining-2-data-frames-join-on-a-common-column) — andrew_reece, Aug 25 '17 at 23:39

score 3 · Accepted Answer · answered Aug 25 '17 at 23:56

Because key has the same value on every row, you'll end up with a Cartesian join because it's unclear which 1 to match on for each key = 1. Instead, match with both key and index.

With:

df1 = pd.DataFrame({"key":np.ones(3, dtype=int), "columnA":np.arange(3)})
df2 = pd.DataFrame({"key":np.ones(3, dtype=int), "columnB":np.arange(3,9,2)})

Merge like this:

df1.merge(df2, on="key", left_index=True, right_index=True)
   columnA  key  columnB
0        0    1        3
1        1    1        5
2        2    1        7

Combine 2 dataframes share the same column in pandas

1 Answers1