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06-Combining-DataFrames

Updated
17 Sept 2026
2 min read
14 sections
Page 19 of 117
Python & Machine Learning

Combining DataFrames

Full Official Guide (Lots of examples!)

https://pandas.pydata.org/pandas-docs/stable/user_guide/merging.html

Concatenation

Directly "glue" together dataframes.

python
data_one = {'A': ['A0', 'A1', 'A2', 'A3'],'B': ['B0', 'B1', 'B2', 'B3']}
python
data_two = {'C': ['C0', 'C1', 'C2', 'C3'], 'D': ['D0', 'D1', 'D2', 'D3']}
python
one = pd.DataFrame(data_one)
python
two = pd.DataFrame(data_two)
python
one
Out · table
A
B
0
A0
B0
9 more lines
1
A1
B1
2
A2
B2
3
A3
B3
python
two
Out · table
C
D
0
C0
D0
9 more lines
1
C1
D1
2
C2
D2
3
C3
D3

Axis = 0

Concatenate along rows

python
axis0 = pd.concat([one,two],axis=0)
python
axis0
Out · table
A
B
C
D
0
39 more lines
A0
B0
NaN
NaN
1
A1
B1
NaN
NaN
2
A2
B2
NaN
NaN
3
A3
B3
NaN
NaN
0
NaN
NaN
C0
D0
1
NaN
NaN
C1
D1
2
NaN
NaN
C2
D2
3
NaN
NaN
C3
D3

Axis = 1

Concatenate along columns

python
axis1 = pd.concat([one,two],axis=1)
python
axis1
Out · table
A
B
C
D
0
19 more lines
A0
B0
C0
D0
1
A1
B1
C1
D1
2
A2
B2
C2
D2
3
A3
B3
C3
D3

Axis 0 , but columns match up

In case you wanted this:

python
two.columns = one.columns
python
pd.concat([one,two])
Out · table
A
B
0
A0
B0
21 more lines
1
A1
B1
2
A2
B2
3
A3
B3
0
C0
D0
1
C1
D1
2
C2
D2
3
C3
D3

Merge

Data Tables

python
registrations = pd.DataFrame({'reg_id':[1,2,3,4],'name':['Andrew','Bobo','Claire','David']})
logins = pd.DataFrame({'log_id':[1,2,3,4],'name':['Xavier','Andrew','Yolanda','Bobo']})
python
registrations
Out · table
reg_id
name
0
1
Andrew
9 more lines
1
2
Bobo
2
3
Claire
3
4
David
python
logins
Out · table
log_id
name
0
1
Xavier
9 more lines
1
2
Andrew
2
3
Yolanda
3
4
Bobo

pd.merge()

Merge pandas DataFrames based on key columns, similar to a SQL join. Results based on the how parameter.

python
help(pd.merge)
Out · stdout
Help on function merge in module pandas.core.reshape.merge:

merge(left, right, how: str = 'inner', on=None, left_on=None, right_on=None, left_index: bool = False, right_index: bool = False, sort: bool = False, suffixes=('_x', '_y'), copy: bool = True, indicator: bool = False, validate=None) -> 'DataFrame'
    Merge DataFrame or named Series objects with a database-style join.
141 more lines
    The join is done on columns or indexes. If joining columns on
    columns, the DataFrame indexes *will be ignored*. Otherwise if joining indexes
    on indexes or indexes on a column or columns, the index will be passed on.
    
    Parameters
    ----------
    left : DataFrame
    right : DataFrame or named Series
        Object to merge with.
    how : {'left', 'right', 'outer', 'inner'}, default 'inner'
        Type of merge to be performed.
    
        * left: use only keys from left frame, similar to a SQL left outer join;
          preserve key order.
        * right: use only keys from right frame, similar to a SQL right outer join;
          preserve key order.
        * outer: use union of keys from both frames, similar to a SQL full outer
          join; sort keys lexicographically.
        * inner: use intersection of keys from both frames, similar to a SQL inner
          join; preserve the order of the left keys.
    on : label or list
        Column or index level names to join on. These must be found in both
        DataFrames. If `on` is None and not merging on indexes then this defaults
        to the intersection of the columns in both DataFrames.
    left_on : label or list, or array-like
        Column or index level names to join on in the left DataFrame. Can also
        be an array or list of arrays of the length of the left DataFrame.
        These arrays are treated as if they are columns.
    right_on : label or list, or array-like
        Column or index level names to join on in the right DataFrame. Can also
        be an array or list of arrays of the length of the right DataFrame.
        These arrays are treated as if they are columns.
    left_index : bool, default False
        Use the index from the left DataFrame as the join key(s). If it is a
        MultiIndex, the number of keys in the other DataFrame (either the index
        or a number of columns) must match the number of levels.
    right_index : bool, default False
        Use the index from the right DataFrame as the join key. Same caveats as
        left_index.
    sort : bool, default False
        Sort the join keys lexicographically in the result DataFrame. If False,
        the order of the join keys depends on the join type (how keyword).
    suffixes : tuple of (str, str), default ('_x', '_y')
        Suffix to apply to overlapping column names in the left and right
        side, respectively. To raise an exception on overlapping columns use
        (False, False).
    copy : bool, default True
        If False, avoid copy if possible.
    indicator : bool or str, default False
        If True, adds a column to output DataFrame called "_merge" with
        information on the source of each row.
        If string, column with information on source of each row will be added to
        output DataFrame, and column will be named value of string.
        Information column is Categorical-type and takes on a value of "left_only"
        for observations whose merge key only appears in 'left' DataFrame,
        "right_only" for observations whose merge key only appears in 'right'
        DataFrame, and "both" if the observation's merge key is found in both.
    
    validate : str, optional
        If specified, checks if merge is of specified type.
    
        * "one_to_one" or "1:1": check if merge keys are unique in both
          left and right datasets.
        * "one_to_many" or "1:m": check if merge keys are unique in left
          dataset.
        * "many_to_one" or "m:1": check if merge keys are unique in right
          dataset.
        * "many_to_many" or "m:m": allowed, but does not result in checks.
    
        .. versionadded:: 0.21.0
    
    Returns
    -------
    DataFrame
        A DataFrame of the two merged objects.
    
    See Also
    --------
    merge_ordered : Merge with optional filling/interpolation.
    merge_asof : Merge on nearest keys.
    DataFrame.join : Similar method using indices.
    
    Notes
    -----
    Support for specifying index levels as the `on`, `left_on`, and
    `right_on` parameters was added in version 0.23.0
    Support for merging named Series objects was added in version 0.24.0
    
    Examples
    --------
    
    >>> df1 = pd.DataFrame({'lkey': ['foo', 'bar', 'baz', 'foo'],
    ...                     'value': [1, 2, 3, 5]})
    >>> df2 = pd.DataFrame({'rkey': ['foo', 'bar', 'baz', 'foo'],
    ...                     'value': [5, 6, 7, 8]})
    >>> df1
        lkey value
    0   foo      1
    1   bar      2
    2   baz      3
    3   foo      5
    >>> df2
        rkey value
    0   foo      5
    1   bar      6
    2   baz      7
    3   foo      8
    
    Merge df1 and df2 on the lkey and rkey columns. The value columns have
    the default suffixes, _x and _y, appended.
    
    >>> df1.merge(df2, left_on='lkey', right_on='rkey')
      lkey  value_x rkey  value_y
    0  foo        1  foo        5
    1  foo        1  foo        8
    2  foo        5  foo        5
    3  foo        5  foo        8
    4  bar        2  bar        6
    5  baz        3  baz        7
    
    Merge DataFrames df1 and df2 with specified left and right suffixes
    appended to any overlapping columns.
    
    >>> df1.merge(df2, left_on='lkey', right_on='rkey',
    ...           suffixes=('_left', '_right'))
      lkey  value_left rkey  value_right
    0  foo           1  foo            5
    1  foo           1  foo            8
    2  foo           5  foo            5
    3  foo           5  foo            8
    4  bar           2  bar            6
    5  baz           3  baz            7
    
    Merge DataFrames df1 and df2, but raise an exception if the DataFrames have
    any overlapping columns.
    
    >>> df1.merge(df2, left_on='lkey', right_on='rkey', suffixes=(False, False))
    Traceback (most recent call last):
    ...
    ValueError: columns overlap but no suffix specified:
        Index(['value'], dtype='object')

Inner,Left, Right, and Outer Joins

Inner Join

Match up where the key is present in BOTH tables. There should be no NaNs due to the join, since by definition to be part of the Inner Join they need info in both tables. Only Andrew and Bobo both registered and logged in.

python
# Notice pd.merge doesn't take in a list like concat
pd.merge(registrations,logins,how='inner',on='name')
Out · table
reg_id
name
log_id
0
1
6 more lines
Andrew
2
1
2
Bobo
4
python
# Pandas smart enough to figure out key column (on parameter) if only one column name matches up
pd.merge(registrations,logins,how='inner')
Out · table
reg_id
name
log_id
0
1
6 more lines
Andrew
2
1
2
Bobo
4
python
# Pandas reports an error if "on" key column isn't in both dataframes
# pd.merge(registrations,logins,how='inner',on='reg_id')

Left Join

Match up AND include all rows from Left Table. Show everyone who registered on Left Table, if they don't have login info, then fill with NaN.

python
pd.merge(registrations,logins,how='left')
Out · table
reg_id
name
log_id
0
1
14 more lines
Andrew
2.0
1
2
Bobo
4.0
2
3
Claire
NaN
3
4
David
NaN

Right Join

Match up AND include all rows from Right Table. Show everyone who logged in on the Right Table, if they don't have registration info, then fill with NaN.

python
pd.merge(registrations,logins,how='right')
Out · table
reg_id
name
log_id
0
1.0
14 more lines
Andrew
2
1
2.0
Bobo
4
2
NaN
Xavier
1
3
NaN
Yolanda
3

Outer Join

Match up on all info found in either Left or Right Table. Show everyone that's in the Log in table and the registrations table. Fill any missing info with NaN

python
pd.merge(registrations,logins,how='outer')
Out · table
reg_id
name
log_id
0
1.0
22 more lines
Andrew
2.0
1
2.0
Bobo
4.0
2
3.0
Claire
NaN
3
4.0
David
NaN
4
NaN
Xavier
1.0
5
NaN
Yolanda
3.0

Join on Index or Column

Use combinations of left_on,right_on,left_index,right_index to merge a column or index on each other

python
registrations
Out · table
reg_id
name
0
1
Andrew
9 more lines
1
2
Bobo
2
3
Claire
3
4
David
python
logins
Out · table
log_id
name
0
1
Xavier
9 more lines
1
2
Andrew
2
3
Yolanda
3
4
Bobo
python
registrations = registrations.set_index("name")
python
registrations
Out · table
reg_id
name
Andrew
1
Bobo
5 more lines
2
Claire
3
David
4
python
pd.merge(registrations,logins,left_index=True,right_on='name')
Out · table
reg_id
log_id
name
1
1
6 more lines
2
Andrew
3
2
4
Bobo
python
pd.merge(logins,registrations,right_index=True,left_on='name')
Out · table
log_id
name
reg_id
1
2
6 more lines
Andrew
1
3
4
Bobo
2

Dealing with differing key column names in joined tables

python
registrations = registrations.reset_index()
python
registrations
Out · table
name
reg_id
0
Andrew
1
9 more lines
1
Bobo
2
2
Claire
3
3
David
4
python
logins
Out · table
log_id
name
0
1
Xavier
9 more lines
1
2
Andrew
2
3
Yolanda
3
4
Bobo
python
registrations.columns = ['reg_name','reg_id']
python
registrations
Out · table
reg_name
reg_id
0
Andrew
1
9 more lines
1
Bobo
2
2
Claire
3
3
David
4
python
# ERROR
# pd.merge(registrations,logins)
python
pd.merge(registrations,logins,left_on='reg_name',right_on='name')
Out · table
reg_name
reg_id
log_id
name
0
9 more lines
Andrew
1
2
Andrew
1
Bobo
2
4
Bobo
python
pd.merge(registrations,logins,left_on='reg_name',right_on='name').drop('reg_name',axis=1)
Out · table
reg_id
log_id
name
0
1
6 more lines
2
Andrew
1
2
4
Bobo

Pandas automatically tags duplicate columns

python
registrations.columns = ['name','id']
python
logins.columns = ['id','name']
python
registrations
Out · table
name
id
0
Andrew
1
9 more lines
1
Bobo
2
2
Claire
3
3
David
4
python
logins
Out · table
id
name
0
1
Xavier
9 more lines
1
2
Andrew
2
3
Yolanda
3
4
Bobo
python
# _x is for left
# _y is for right
pd.merge(registrations,logins,on='name')
Out · table
name
id_x
id_y
0
Andrew
6 more lines
1
2
1
Bobo
2
4
python
pd.merge(registrations,logins,on='name',suffixes=('_reg','_log'))
Out · table
name
id_reg
id_log
0
Andrew
6 more lines
1
2
1
Bobo
2
4