Difference between Merge, join, and concatenate

The merge() and join() methods are the DataFrame method, not a series method. The concat() method is the pandas’ method which provides the functionality to combine the pandas’ objects such as DataFrame and Series.

Merge – 

• The merge() function used to merge the DataFrames with database-style join such as inner join, outer join, left join, right join.
• Combining exactly two DataFrames.
• The join is done on columns or indexes.
• If joining columns on columns, the DataFrame indexes will be ignored.
• If joining indexes on indexes or indexes on a column, the index will be passed on.

Join – 

• The join() function used to join two or more pandas DataFrames/Series horizontally.
• Join() uses merge internally for the index-on-index (by default) and column(s)-on-index join.
• Aligns the calling DataFrame’s column(s) or index with the other objects’ index (and not the columns).
• Defaults to left join with options for right, inner and outer join

Concat – 

• concatenate two or more pandas DataFrames/Series vertically or horizontally.
• Aligns only on the index by specifying the axis parameter.
• Defaults to outer join with the option for inner join

Syntax

DataFrame.merge(self, right, how='inner', on=None, left_on=None, right_on=None,   left_index=False, right_index=False, sort=False, suffixes=('_x', '_y'), copy=True, indicator=False, validate=None) 

DataFrame.join(self, other, on=None, how='left', lsuffix='', rsuffix='', sort=False) 

pandas.concat(objs, axis=0, join='outer', join_axes=None, ignore_index=False, keys=None, levels=None, names=None, verify_integrity=False, sort=None, copy=True)

Examples 

In [1]:
# Let's Define the DataFrames
import pandas as pd
data1 = {'key1':['k0','k1'], 'name' :['mark','juli'],'city':['New York','Paris']}
data2 = {'key1':['k1','k2'],'name' :['john','alex'],'city':['London','Tokyo']}

df1 = pd.DataFrame(data1,index = [0,1],columns=['key1','city','name'])
df2 = pd.DataFrame(data2,index=[1,'k0'],columns=['key1','city','name'])

Merge() –

In [2]: df1.merge(df2,on="key1")
Out[2]:
  key1 city_x name_x  city_y name_y
0   k1  Paris   juli  London   john

Join() –

In [3]: df1.join(df2, lsuffix='_')
Out[3]:
  key1_     city_ name_ key1    city  name
0    k0  New York  mark  NaN     NaN   NaN
1    k1     Paris  juli   k1  London  john

In [4]: df1.join(df2, on="key1",lsuffix='_')
Out[4]:
  key1_     city_ name_ key1   city  name
0    k0  New York  mark   k2  Tokyo  alex
1    k1     Paris  juli  NaN    NaN   NaN

Concat() –

In [5]: pd.concat([df1,df2])
Out[5]:
   key1      city  name
0    k0  New York  mark
1    k1     Paris  juli
1    k1    London  john
k0   k2     Tokyo  alex

In [6]: pd.concat([df1,df2],axis=1)
Out[6]:
   key1      city  name key1    city  name
0    k0  New York  mark  NaN     NaN   NaN
1    k1     Paris  juli   k1  London  john
k0  NaN       NaN   NaN   k2   Tokyo  alex

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