concatenate and merge two dataframes in Python

# concatenate vertically two or more dataframes
pd.concat([men2004, men2008], ignore_index = False, axis=0, keys = [2004,2008], names = 'Year')

# add (sum indexes): aggregate/add the total number of Gold, Silver and Bronze Medals over both editions
ath_2008.set_index("Athlete", inplace= True)
ath_2012.set_index("Athlete", inplace= True)
add = ath_2008.add(ath_2012, fill_value=0)
add.head(10)

# substract
top1_df.sub(top2_df, fill_value = 0)

# outer join
olimpic1.merge(olimpic2, how = 'outer', on = 'Athlete', suffixes = ('_2005','_2020'), indicator = True)

# inner join
olimpic1.merge(olimpic2, how = 'inner', on = 'Athlete', suffixes = ('_2005','_2020'), indicator = True)

# outer join without intersection
combo_df = olimpic1.merge(olimpic2, how = 'outer', on = 'Athlete', suffixes = ('_2005','_2020'), indicator = True)
combo_df.loc[combo_df._merge != 'both']

# left join without intersection
combo_df = olimpic1.merge(olimpic2, how = 'outer', on = 'Athlete', suffixes = ('_2005','_2020'), indicator = True)
combo_df.loc[combo_df._merge == 'left_only']

# right join without intersection
combo_df = olimpic1.merge(olimpic2, how = 'outer', on = 'Athlete', suffixes = ('_2005','_2020'), indicator = True)
combo_df.loc[combo_df._merge == 'right_only']

# left join
combo_df = olimpic1.merge(olimpic2, how = 'left', on = 'Athlete', suffixes = ('_2005','_2020'), indicator = True)

# join on more than one column
combo_df = olimpic1.merge(olimpic2, how = 'outer', on = ['Athlete', 'Medal'], suffixes = ('_2005','_2020'), indicator = True)
combo_df = olimpic1.merge(olimpic2, how = 'inner', on = ['Athlete', 'Medal'], suffixes = ('_2005','_2020'), indicator = True)