Join pandas dataframes based on column values

Question

I'm quite new to pandas dataframes, and I'm experiencing some troubles joining two tables.

The first df has just 3 columns:

DF1:
item_id    position    document_id
336        1           10
337        2           10
338        3           10
1001       1           11
1002       2           11
1003       3           11
38         10          146

And the second has exactly same two columns (and plenty of others):

DF2
item_id    document_id    col1    col2   col3    ...
337        10             ...     ...    ...
1002       11             ...     ...    ...
1003       11             ...     ...    ...

What I need is to perform an operation which, in SQL, would look as follows:

DF1 join DF2 on 
DF1.document_id = DF2.document_id
and
DF1.item_id = DF2.item_id

And, as a result, I want to see DF2, complemented with column 'position':

item_id    document_id    position    col1   col2   col3   ...

What is a good way to do this using pandas?

Thank you!

Answer 1

I think you need merge with default inner join, but is necessary no duplicated combinations of values in both columns:

print (df2)
   item_id  document_id col1  col2  col3
0      337           10    s     4     7
1     1002           11    d     5     8
2     1003           11    f     7     0

df = pd.merge(df1, df2, on=['document_id','item_id'])
print (df)
   item_id  position  document_id col1  col2  col3
0      337         2           10    s     4     7
1     1002         2           11    d     5     8
2     1003         3           11    f     7     0

But if necessary position column in position 3:

df = pd.merge(df2, df1, on=['document_id','item_id'])
cols = df.columns.tolist()
df = df[cols[:2] + cols[-1:] + cols[2:-1]]
print (df)
   item_id  document_id  position col1  col2  col3
0      337           10         2    s     4     7
1     1002           11         2    d     5     8
2     1003           11         3    f     7     0