.. currentmodule:: pandas

.. ipython:: python
   :suppress:

   import numpy as np
   np.random.seed(123456)
   import pandas as pd
   pd.options.display.max_rows=15
   randn = np.random.randn
   np.set_printoptions(precision=4, suppress=True)

   import matplotlib.pyplot as plt
   plt.close('all')
   import pandas.util._doctools as doctools
   p = doctools.TablePlotter()

Merge, join, and concatenate

pandas provides various facilities for easily combining together Series, DataFrame, and Panel objects with various kinds of set logic for the indexes and relational algebra functionality in the case of join / merge-type operations.

Concatenating objects

The :func:`~pandas.concat` function (in the main pandas namespace) does all of the heavy lifting of performing concatenation operations along an axis while performing optional set logic (union or intersection) of the indexes (if any) on the other axes. Note that I say "if any" because there is only a single possible axis of concatenation for Series.

Before diving into all of the details of concat and what it can do, here is a simple example:

.. ipython:: python

   df1 = pd.DataFrame({'A': ['A0', 'A1', 'A2', 'A3'],
                       'B': ['B0', 'B1', 'B2', 'B3'],
                       'C': ['C0', 'C1', 'C2', 'C3'],
                       'D': ['D0', 'D1', 'D2', 'D3']},
                       index=[0, 1, 2, 3])

   df2 = pd.DataFrame({'A': ['A4', 'A5', 'A6', 'A7'],
                       'B': ['B4', 'B5', 'B6', 'B7'],
                       'C': ['C4', 'C5', 'C6', 'C7'],
                       'D': ['D4', 'D5', 'D6', 'D7']},
                        index=[4, 5, 6, 7])

   df3 = pd.DataFrame({'A': ['A8', 'A9', 'A10', 'A11'],
                       'B': ['B8', 'B9', 'B10', 'B11'],
                       'C': ['C8', 'C9', 'C10', 'C11'],
                       'D': ['D8', 'D9', 'D10', 'D11']},
                       index=[8, 9, 10, 11])

   frames = [df1, df2, df3]
   result = pd.concat(frames)

.. ipython:: python
   :suppress:

   @savefig merging_concat_basic.png
   p.plot(frames, result,
          labels=['df1', 'df2', 'df3'], vertical=True);
   plt.close('all');

Like its sibling function on ndarrays, numpy.concatenate, pandas.concat takes a list or dict of homogeneously-typed objects and concatenates them with some configurable handling of "what to do with the other axes":

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

objs : a sequence or mapping of Series, DataFrame, or Panel objects. If a dict is passed, the sorted keys will be used as the keys argument, unless it is passed, in which case the values will be selected (see below). Any None objects will be dropped silently unless they are all None in which case a ValueError will be raised.
axis : {0, 1, ...}, default 0. The axis to concatenate along.
join : {'inner', 'outer'}, default 'outer'. How to handle indexes on other axis(es). Outer for union and inner for intersection.
ignore_index : boolean, default False. If True, do not use the index values on the concatenation axis. The resulting axis will be labeled 0, ..., n - 1. This is useful if you are concatenating objects where the concatenation axis does not have meaningful indexing information. Note the index values on the other axes are still respected in the join.
join_axes : list of Index objects. Specific indexes to use for the other n - 1 axes instead of performing inner/outer set logic.
keys : sequence, default None. Construct hierarchical index using the passed keys as the outermost level. If multiple levels passed, should contain tuples.
levels : list of sequences, default None. Specific levels (unique values) to use for constructing a MultiIndex. Otherwise they will be inferred from the keys.
names : list, default None. Names for the levels in the resulting hierarchical index.
verify_integrity : boolean, default False. Check whether the new concatenated axis contains duplicates. This can be very expensive relative to the actual data concatenation.
copy : boolean, default True. If False, do not copy data unnecessarily.

Without a little bit of context many of these arguments don't make much sense. Let's revisit the above example. Suppose we wanted to associate specific keys with each of the pieces of the chopped up DataFrame. We can do this using the keys argument:

.. ipython:: python

   result = pd.concat(frames, keys=['x', 'y', 'z'])

.. ipython:: python
   :suppress:

   @savefig merging_concat_keys.png
   p.plot(frames, result,
          labels=['df1', 'df2', 'df3'], vertical=True)
   plt.close('all');

As you can see (if you've read the rest of the documentation), the resulting object's index has a :ref:`hierarchical index <advanced.hierarchical>`. This means that we can now select out each chunk by key:

.. ipython:: python

   result.loc['y']

It's not a stretch to see how this can be very useful. More detail on this functionality below.

Note

It is worth noting that :func:`~pandas.concat` (and therefore :func:`~pandas.append`) makes a full copy of the data, and that constantly reusing this function can create a significant performance hit. If you need to use the operation over several datasets, use a list comprehension.

frames = [ process_your_file(f) for f in files ]
result = pd.concat(frames)

Set logic on the other axes

When gluing together multiple DataFrames, you have a choice of how to handle the other axes (other than the one being concatenated). This can be done in the following three ways:

Take the (sorted) union of them all, join='outer'. This is the default option as it results in zero information loss.
Take the intersection, join='inner'.
Use a specific index, as passed to the join_axes argument.

Here is an example of each of these methods. First, the default join='outer' behavior:

.. ipython:: python

   df4 = pd.DataFrame({'B': ['B2', 'B3', 'B6', 'B7'],
                    'D': ['D2', 'D3', 'D6', 'D7'],
                    'F': ['F2', 'F3', 'F6', 'F7']},
                   index=[2, 3, 6, 7])
   result = pd.concat([df1, df4], axis=1)

.. ipython:: python
   :suppress:

   @savefig merging_concat_axis1.png
   p.plot([df1, df4], result,
          labels=['df1', 'df4'], vertical=False);
   plt.close('all');

Note that the row indexes have been unioned and sorted. Here is the same thing with join='inner':

.. ipython:: python

   result = pd.concat([df1, df4], axis=1, join='inner')

.. ipython:: python
   :suppress:

   @savefig merging_concat_axis1_inner.png
   p.plot([df1, df4], result,
          labels=['df1', 'df4'], vertical=False);
   plt.close('all');

Lastly, suppose we just wanted to reuse the exact index from the original DataFrame:

.. ipython:: python

   result = pd.concat([df1, df4], axis=1, join_axes=[df1.index])

.. ipython:: python
   :suppress:

   @savefig merging_concat_axis1_join_axes.png
   p.plot([df1, df4], result,
          labels=['df1', 'df4'], vertical=False);
   plt.close('all');

Concatenating using `append`

A useful shortcut to :func:`~pandas.concat` are the :meth:`~DataFrame.append` instance methods on Series and DataFrame. These methods actually predated concat. They concatenate along axis=0, namely the index:

.. ipython:: python

   result = df1.append(df2)

.. ipython:: python
   :suppress:

   @savefig merging_append1.png
   p.plot([df1, df2], result,
          labels=['df1', 'df2'], vertical=True);
   plt.close('all');

In the case of DataFrame, the indexes must be disjoint but the columns do not need to be:

.. ipython:: python

   result = df1.append(df4)

.. ipython:: python
   :suppress:

   @savefig merging_append2.png
   p.plot([df1, df4], result,
          labels=['df1', 'df4'], vertical=True);
   plt.close('all');

append may take multiple objects to concatenate:

.. ipython:: python

   result = df1.append([df2, df3])

.. ipython:: python
   :suppress:

   @savefig merging_append3.png
   p.plot([df1, df2, df3], result,
          labels=['df1', 'df2', 'df3'], vertical=True);
   plt.close('all');

Note

Unlike the :py:meth:`~list.append` method, which appends to the original list and returns None, :meth:`~DataFrame.append` here does not modify df1 and returns its copy with df2 appended.

Ignoring indexes on the concatenation axis

For DataFrame``s which don't have a meaningful index, you may wish to append them and ignore the fact that they may have overlapping indexes. To do this, use the ``ignore_index argument:

.. ipython:: python

   result = pd.concat([df1, df4], ignore_index=True)

.. ipython:: python
   :suppress:

   @savefig merging_concat_ignore_index.png
   p.plot([df1, df4], result,
          labels=['df1', 'df4'], vertical=True);
   plt.close('all');

This is also a valid argument to :meth:`DataFrame.append`:

.. ipython:: python

   result = df1.append(df4, ignore_index=True)

.. ipython:: python
   :suppress:

   @savefig merging_append_ignore_index.png
   p.plot([df1, df4], result,
          labels=['df1', 'df4'], vertical=True);
   plt.close('all');

Concatenating with mixed ndims

You can concatenate a mix of Series and DataFrame``s. The ``Series will be transformed to DataFrame with the column name as the name of the Series.

.. ipython:: python

   s1 = pd.Series(['X0', 'X1', 'X2', 'X3'], name='X')
   result = pd.concat([df1, s1], axis=1)

.. ipython:: python
   :suppress:

   @savefig merging_concat_mixed_ndim.png
   p.plot([df1, s1], result,
          labels=['df1', 's1'], vertical=False);
   plt.close('all');

Note

Since we're concatenating a Series to a DataFrame, we could have achieved the same result with :meth:`DataFrame.assign`. To concatenate an arbitrary number of pandas objects (DataFrame or Series), use concat.

If unnamed Series are passed they will be numbered consecutively.

.. ipython:: python

   s2 = pd.Series(['_0', '_1', '_2', '_3'])
   result = pd.concat([df1, s2, s2, s2], axis=1)

.. ipython:: python
   :suppress:

   @savefig merging_concat_unnamed_series.png
   p.plot([df1, s2], result,
          labels=['df1', 's2'], vertical=False);
   plt.close('all');

Passing ignore_index=True will drop all name references.

.. ipython:: python

   result = pd.concat([df1, s1], axis=1, ignore_index=True)

.. ipython:: python
   :suppress:

   @savefig merging_concat_series_ignore_index.png
   p.plot([df1, s1], result,
          labels=['df1', 's1'], vertical=False);
   plt.close('all');

More concatenating with group keys

A fairly common use of the keys argument is to override the column names when creating a new DataFrame based on existing Series. Notice how the default behaviour consists on letting the resulting DataFrame inherit the parent Series' name, when these existed.

.. ipython:: python

   s3 = pd.Series([0, 1, 2, 3], name='foo')
   s4 = pd.Series([0, 1, 2, 3])
   s5 = pd.Series([0, 1, 4, 5])

   pd.concat([s3, s4, s5], axis=1)

Through the keys argument we can override the existing column names.

.. ipython:: python

   pd.concat([s3, s4, s5], axis=1, keys=['red','blue','yellow'])

Let's consider a variation of the very first example presented:

.. ipython:: python

   result = pd.concat(frames, keys=['x', 'y', 'z'])

.. ipython:: python
   :suppress:

   @savefig merging_concat_group_keys2.png
   p.plot(frames, result,
          labels=['df1', 'df2', 'df3'], vertical=True);
   plt.close('all');

You can also pass a dict to concat in which case the dict keys will be used for the keys argument (unless other keys are specified):

.. ipython:: python

   pieces = {'x': df1, 'y': df2, 'z': df3}
   result = pd.concat(pieces)

.. ipython:: python
   :suppress:

   @savefig merging_concat_dict.png
   p.plot([df1, df2, df3], result,
          labels=['df1', 'df2', 'df3'], vertical=True);
   plt.close('all');

.. ipython:: python

   result = pd.concat(pieces, keys=['z', 'y'])

.. ipython:: python
   :suppress:

   @savefig merging_concat_dict_keys.png
   p.plot([df1, df2, df3], result,
          labels=['df1', 'df2', 'df3'], vertical=True);
   plt.close('all');

The MultiIndex created has levels that are constructed from the passed keys and the index of the DataFrame pieces:

.. ipython:: python

   result.index.levels

If you wish to specify other levels (as will occasionally be the case), you can do so using the levels argument:

.. ipython:: python

   result = pd.concat(pieces, keys=['x', 'y', 'z'],
                   levels=[['z', 'y', 'x', 'w']],
                   names=['group_key'])

.. ipython:: python
   :suppress:

   @savefig merging_concat_dict_keys_names.png
   p.plot([df1, df2, df3], result,
          labels=['df1', 'df2', 'df3'], vertical=True);
   plt.close('all');

.. ipython:: python

   result.index.levels

This is fairly esoteric, but it is actually necessary for implementing things like GroupBy where the order of a categorical variable is meaningful.

Appending rows to a DataFrame

While not especially efficient (since a new object must be created), you can append a single row to a DataFrame by passing a Series or dict to append, which returns a new DataFrame as above.

.. ipython:: python

   s2 = pd.Series(['X0', 'X1', 'X2', 'X3'], index=['A', 'B', 'C', 'D'])
   result = df1.append(s2, ignore_index=True)

.. ipython:: python
   :suppress:

   @savefig merging_append_series_as_row.png
   p.plot([df1, s2], result,
          labels=['df1', 's2'], vertical=True);
   plt.close('all');

You should use ignore_index with this method to instruct DataFrame to discard its index. If you wish to preserve the index, you should construct an appropriately-indexed DataFrame and append or concatenate those objects.

You can also pass a list of dicts or Series:

.. ipython:: python

   dicts = [{'A': 1, 'B': 2, 'C': 3, 'X': 4},
            {'A': 5, 'B': 6, 'C': 7, 'Y': 8}]
   result = df1.append(dicts, ignore_index=True)

.. ipython:: python
   :suppress:

   @savefig merging_append_dits.png
   p.plot([df1, pd.DataFrame(dicts)], result,
          labels=['df1', 'dicts'], vertical=True);
   plt.close('all');

Database-style DataFrame joining/merging

pandas has full-featured, high performance in-memory join operations idiomatically very similar to relational databases like SQL. These methods perform significantly better (in some cases well over an order of magnitude better) than other open source implementations (like base::merge.data.frame in R). The reason for this is careful algorithmic design and the internal layout of the data in DataFrame.

See the :ref:`cookbook<cookbook.merge>` for some advanced strategies.

Users who are familiar with SQL but new to pandas might be interested in a :ref:`comparison with SQL<compare_with_sql.join>`.

pandas provides a single function, :func:`~pandas.merge`, as the entry point for all standard database join operations between DataFrame objects:

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

left: A DataFrame object.
right: Another DataFrame object.
on: Column or index level names to join on. Must be found in both the left and right DataFrame objects. If not passed and left_index and right_index are False, the intersection of the columns in the DataFrames will be inferred to be the join keys.
left_on: Columns or index levels from the left DataFrame to use as keys. Can either be column names, index level names, or arrays with length equal to the length of the DataFrame.
right_on: Columns or index levels from the right DataFrame to use as keys. Can either be column names, index level names, or arrays with length equal to the length of the DataFrame.
left_index: If True, use the index (row labels) from the left DataFrame as its join key(s). In the case of a DataFrame with a MultiIndex (hierarchical), the number of levels must match the number of join keys from the right DataFrame.
right_index: Same usage as left_index for the right DataFrame
how: One of 'left', 'right', 'outer', 'inner'. Defaults to inner. See below for more detailed description of each method.
sort: Sort the result DataFrame by the join keys in lexicographical order. Defaults to True, setting to False will improve performance substantially in many cases.
suffixes: A tuple of string suffixes to apply to overlapping columns. Defaults to ('_x', '_y').
copy: Always copy data (default True) from the passed DataFrame objects, even when reindexing is not necessary. Cannot be avoided in many cases but may improve performance / memory usage. The cases where copying can be avoided are somewhat pathological but this option is provided nonetheless.
indicator: Add a column to the output DataFrame called _merge with information on the source of each row. _merge 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 : string, default None. If specified, checks if merge is of specified type.
- "one_to_one" or "1:1": checks if merge keys are unique in both left and right datasets.
- "one_to_many" or "1:m": checks if merge keys are unique in left dataset.
- "many_to_one" or "m:1": checks 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
```

Note

Support for specifying index levels as the on, left_on, and right_on parameters was added in version 0.23.0.

The return type will be the same as left. If left is a DataFrame and right is a subclass of DataFrame, the return type will still be DataFrame.

merge is a function in the pandas namespace, and it is also available as a DataFrame instance method :meth:`~DataFrame.merge`, with the calling ``DataFrame `` being implicitly considered the left object in the join.

The related :meth:`~DataFrame.join` method, uses merge internally for the index-on-index (by default) and column(s)-on-index join. If you are joining on index only, you may wish to use DataFrame.join to save yourself some typing.

Brief primer on merge methods (relational algebra)

Experienced users of relational databases like SQL will be familiar with the terminology used to describe join operations between two SQL-table like structures (DataFrame objects). There are several cases to consider which are very important to understand:

one-to-one joins: for example when joining two DataFrame objects on their indexes (which must contain unique values).
many-to-one joins: for example when joining an index (unique) to one or more columns in a different DataFrame.
many-to-many joins: joining columns on columns.

Note

When joining columns on columns (potentially a many-to-many join), any indexes on the passed DataFrame objects will be discarded.

It is worth spending some time understanding the result of the many-to-many join case. In SQL / standard relational algebra, if a key combination appears more than once in both tables, the resulting table will have the Cartesian product of the associated data. Here is a very basic example with one unique key combination:

.. ipython:: python

   left = pd.DataFrame({'key': ['K0', 'K1', 'K2', 'K3'],
                        'A': ['A0', 'A1', 'A2', 'A3'],
                        'B': ['B0', 'B1', 'B2', 'B3']})

   right = pd.DataFrame({'key': ['K0', 'K1', 'K2', 'K3'],
                         'C': ['C0', 'C1', 'C2', 'C3'],
                         'D': ['D0', 'D1', 'D2', 'D3']})
   result = pd.merge(left, right, on='key')

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Here is a more complicated example with multiple join keys. Only the keys appearing in left and right are present (the intersection), since how='inner' by default.

.. ipython:: python

   left = pd.DataFrame({'key1': ['K0', 'K0', 'K1', 'K2'],
                        'key2': ['K0', 'K1', 'K0', 'K1'],
                        'A': ['A0', 'A1', 'A2', 'A3'],
                        'B': ['B0', 'B1', 'B2', 'B3']})

   right = pd.DataFrame({'key1': ['K0', 'K1', 'K1', 'K2'],
                         'key2': ['K0', 'K0', 'K0', 'K0'],
                         'C': ['C0', 'C1', 'C2', 'C3'],
                         'D': ['D0', 'D1', 'D2', 'D3']})

   result = pd.merge(left, right, on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key_multiple.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

The how argument to merge specifies how to determine which keys are to be included in the resulting table. If a key combination does not appear in either the left or right tables, the values in the joined table will be NA. Here is a summary of the how options and their SQL equivalent names:

Merge method	SQL Join Name	Description
`left`	`LEFT OUTER JOIN`	Use keys from left frame only
`right`	`RIGHT OUTER JOIN`	Use keys from right frame only
`outer`	`FULL OUTER JOIN`	Use union of keys from both frames
`inner`	`INNER JOIN`	Use intersection of keys from both frames

.. ipython:: python

   result = pd.merge(left, right, how='left', on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key_left.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

.. ipython:: python

   result = pd.merge(left, right, how='right', on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key_right.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);

.. ipython:: python

   result = pd.merge(left, right, how='outer', on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key_outer.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

.. ipython:: python

   result = pd.merge(left, right, how='inner', on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key_inner.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Here is another example with duplicate join keys in DataFrames:

.. ipython:: python

   left = pd.DataFrame({'A' : [1,2], 'B' : [2, 2]})

   right = pd.DataFrame({'A' : [4,5,6], 'B': [2,2,2]})

   result = pd.merge(left, right, on='B', how='outer')

.. ipython:: python
   :suppress:

   @savefig merging_merge_on_key_dup.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Warning

Joining / merging on duplicate keys can cause a returned frame that is the multiplication of the row dimensions, which may result in memory overflow. It is the user' s responsibility to manage duplicate values in keys before joining large DataFrames.

Checking for duplicate keys

.. versionadded:: 0.21.0

Users can use the validate argument to automatically check whether there are unexpected duplicates in their merge keys. Key uniqueness is checked before merge operations and so should protect against memory overflows. Checking key uniqueness is also a good way to ensure user data structures are as expected.

In the following example, there are duplicate values of B in the right DataFrame. As this is not a one-to-one merge -- as specified in the validate argument -- an exception will be raised.

.. ipython:: python

  left = pd.DataFrame({'A' : [1,2], 'B' : [1, 2]})
  right = pd.DataFrame({'A' : [4,5,6], 'B': [2, 2, 2]})

In [53]: result = pd.merge(left, right, on='B', how='outer', validate="one_to_one")
...
MergeError: Merge keys are not unique in right dataset; not a one-to-one merge

If the user is aware of the duplicates in the right DataFrame but wants to ensure there are no duplicates in the left DataFrame, one can use the validate='one_to_many' argument instead, which will not raise an exception.

.. ipython:: python

   pd.merge(left, right, on='B', how='outer', validate="one_to_many")

The merge indicator

:func:`~pandas.merge` accepts the argument indicator. If True, a Categorical-type column called _merge will be added to the output object that takes on values:

Observation Origin _merge value

Merge key only in 'left' frame left_only

Merge key only in 'right' frame right_only

Merge key in both frames both

.. ipython:: python

   df1 = pd.DataFrame({'col1': [0, 1], 'col_left':['a', 'b']})
   df2 = pd.DataFrame({'col1': [1, 2, 2],'col_right':[2, 2, 2]})
   pd.merge(df1, df2, on='col1', how='outer', indicator=True)

The indicator argument will also accept string arguments, in which case the indicator function will use the value of the passed string as the name for the indicator column.

.. ipython:: python

   pd.merge(df1, df2, on='col1', how='outer', indicator='indicator_column')

Merge Dtypes

.. versionadded:: 0.19.0

Merging will preserve the dtype of the join keys.

.. ipython:: python

   left = pd.DataFrame({'key': [1], 'v1': [10]})
   left
   right = pd.DataFrame({'key': [1, 2], 'v1': [20, 30]})
   right

We are able to preserve the join keys:

.. ipython:: python

   pd.merge(left, right, how='outer')
   pd.merge(left, right, how='outer').dtypes

Of course if you have missing values that are introduced, then the resulting dtype will be upcast.

.. ipython:: python

   pd.merge(left, right, how='outer', on='key')
   pd.merge(left, right, how='outer', on='key').dtypes

.. versionadded:: 0.20.0

Merging will preserve category dtypes of the mergands. See also the section on :ref:`categoricals <categorical.merge>`.

The left frame.

.. ipython:: python

   from pandas.api.types import CategoricalDtype

   X = pd.Series(np.random.choice(['foo', 'bar'], size=(10,)))
   X = X.astype(CategoricalDtype(categories=['foo', 'bar']))

   left = pd.DataFrame({'X': X,
                        'Y': np.random.choice(['one', 'two', 'three'], size=(10,))})
   left
   left.dtypes

The right frame.

.. ipython:: python

   right = pd.DataFrame({
        'X': pd.Series(['foo', 'bar'],
                       dtype=CategoricalDtype(['foo', 'bar'])),
        'Z': [1, 2]
   })
   right
   right.dtypes

The merged result:

.. ipython:: python

   result = pd.merge(left, right, how='outer')
   result
   result.dtypes

Note

The category dtypes must be exactly the same, meaning the same categories and the ordered attribute. Otherwise the result will coerce to object dtype.

Note

Merging on category dtypes that are the same can be quite performant compared to object dtype merging.

Joining on index

:meth:`DataFrame.join` is a convenient method for combining the columns of two potentially differently-indexed DataFrames into a single result DataFrame. Here is a very basic example:

.. ipython:: python

   left = pd.DataFrame({'A': ['A0', 'A1', 'A2'],
                        'B': ['B0', 'B1', 'B2']},
                        index=['K0', 'K1', 'K2'])

   right = pd.DataFrame({'C': ['C0', 'C2', 'C3'],
                         'D': ['D0', 'D2', 'D3']},
                         index=['K0', 'K2', 'K3'])

   result = left.join(right)

.. ipython:: python
   :suppress:

   @savefig merging_join.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

.. ipython:: python

   result = left.join(right, how='outer')

.. ipython:: python
   :suppress:

   @savefig merging_join_outer.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

The same as above, but with how='inner'.

.. ipython:: python

   result = left.join(right, how='inner')

.. ipython:: python
   :suppress:

   @savefig merging_join_inner.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

The data alignment here is on the indexes (row labels). This same behavior can be achieved using merge plus additional arguments instructing it to use the indexes:

.. ipython:: python

   result = pd.merge(left, right, left_index=True, right_index=True, how='outer')

.. ipython:: python
   :suppress:

   @savefig merging_merge_index_outer.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

.. ipython:: python

   result = pd.merge(left, right, left_index=True, right_index=True, how='inner');

.. ipython:: python
   :suppress:

   @savefig merging_merge_index_inner.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Joining key columns on an index

:meth:`~DataFrame.join` takes an optional on argument which may be a column or multiple column names, which specifies that the passed DataFrame is to be aligned on that column in the DataFrame. These two function calls are completely equivalent:

left.join(right, on=key_or_keys)
pd.merge(left, right, left_on=key_or_keys, right_index=True,
      how='left', sort=False)

Obviously you can choose whichever form you find more convenient. For many-to-one joins (where one of the DataFrame's is already indexed by the join key), using join may be more convenient. Here is a simple example:

.. ipython:: python

   left = pd.DataFrame({'A': ['A0', 'A1', 'A2', 'A3'],
                        'B': ['B0', 'B1', 'B2', 'B3'],
                        'key': ['K0', 'K1', 'K0', 'K1']})

   right = pd.DataFrame({'C': ['C0', 'C1'],
                         'D': ['D0', 'D1']},
                         index=['K0', 'K1'])

   result = left.join(right, on='key')

.. ipython:: python
   :suppress:

   @savefig merging_join_key_columns.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

.. ipython:: python

   result = pd.merge(left, right, left_on='key', right_index=True,
                     how='left', sort=False);

.. ipython:: python
   :suppress:

   @savefig merging_merge_key_columns.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

To join on multiple keys, the passed DataFrame must have a MultiIndex:

.. ipython:: python

   left = pd.DataFrame({'A': ['A0', 'A1', 'A2', 'A3'],
                        'B': ['B0', 'B1', 'B2', 'B3'],
                        'key1': ['K0', 'K0', 'K1', 'K2'],
                        'key2': ['K0', 'K1', 'K0', 'K1']})

   index = pd.MultiIndex.from_tuples([('K0', 'K0'), ('K1', 'K0'),
                                     ('K2', 'K0'), ('K2', 'K1')])
   right = pd.DataFrame({'C': ['C0', 'C1', 'C2', 'C3'],
                      'D': ['D0', 'D1', 'D2', 'D3']},
                     index=index)

Now this can be joined by passing the two key column names:

.. ipython:: python

   result = left.join(right, on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merging_join_multikeys.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

The default for DataFrame.join is to perform a left join (essentially a "VLOOKUP" operation, for Excel users), which uses only the keys found in the calling DataFrame. Other join types, for example inner join, can be just as easily performed:

.. ipython:: python

   result = left.join(right, on=['key1', 'key2'], how='inner')

.. ipython:: python
   :suppress:

   @savefig merging_join_multikeys_inner.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

As you can see, this drops any rows where there was no match.

Joining a single Index to a Multi-index

You can join a singly-indexed DataFrame with a level of a multi-indexed DataFrame. The level will match on the name of the index of the singly-indexed frame against a level name of the multi-indexed frame.

..  ipython:: python

   left = pd.DataFrame({'A': ['A0', 'A1', 'A2'],
                        'B': ['B0', 'B1', 'B2']},
                        index=pd.Index(['K0', 'K1', 'K2'], name='key'))

   index = pd.MultiIndex.from_tuples([('K0', 'Y0'), ('K1', 'Y1'),
                                     ('K2', 'Y2'), ('K2', 'Y3')],
                                      names=['key', 'Y'])
   right = pd.DataFrame({'C': ['C0', 'C1', 'C2', 'C3'],
                         'D': ['D0', 'D1', 'D2', 'D3']},
                         index=index)

   result = left.join(right, how='inner')

.. ipython:: python
   :suppress:

   @savefig merging_join_multiindex_inner.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

This is equivalent but less verbose and more memory efficient / faster than this.

..  ipython:: python

    result = pd.merge(left.reset_index(), right.reset_index(),
          on=['key'], how='inner').set_index(['key','Y'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_multiindex_alternative.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Joining with two multi-indexes

This is supported in a limited way, provided that the index for the right argument is completely used in the join, and is a subset of the indices in the left argument, as in this example:

.. ipython:: python

   leftindex = pd.MultiIndex.from_product([list('abc'), list('xy'), [1, 2]],
                                          names=['abc', 'xy', 'num'])
   left = pd.DataFrame({'v1' : range(12)}, index=leftindex)
   left

   rightindex = pd.MultiIndex.from_product([list('abc'), list('xy')],
                                           names=['abc', 'xy'])
   right = pd.DataFrame({'v2': [100*i for i in range(1, 7)]}, index=rightindex)
   right

   left.join(right, on=['abc', 'xy'], how='inner')

If that condition is not satisfied, a join with two multi-indexes can be done using the following code.

.. ipython:: python

   leftindex = pd.MultiIndex.from_tuples([('K0', 'X0'), ('K0', 'X1'),
                                          ('K1', 'X2')],
                                         names=['key', 'X'])
   left = pd.DataFrame({'A': ['A0', 'A1', 'A2'],
                        'B': ['B0', 'B1', 'B2']},
                        index=leftindex)

   rightindex = pd.MultiIndex.from_tuples([('K0', 'Y0'), ('K1', 'Y1'),
                                           ('K2', 'Y2'), ('K2', 'Y3')],
                                          names=['key', 'Y'])
   right = pd.DataFrame({'C': ['C0', 'C1', 'C2', 'C3'],
                         'D': ['D0', 'D1', 'D2', 'D3']},
                         index=rightindex)

   result = pd.merge(left.reset_index(), right.reset_index(),
                     on=['key'], how='inner').set_index(['key','X','Y'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_two_multiindex.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Merging on a combination of columns and index levels

.. versionadded:: 0.23

Strings passed as the on, left_on, and right_on parameters may refer to either column names or index level names. This enables merging DataFrame instances on a combination of index levels and columns without resetting indexes.

.. ipython:: python

   left_index = pd.Index(['K0', 'K0', 'K1', 'K2'], name='key1')

   left = pd.DataFrame({'A': ['A0', 'A1', 'A2', 'A3'],
                        'B': ['B0', 'B1', 'B2', 'B3'],
                        'key2': ['K0', 'K1', 'K0', 'K1']},
                       index=left_index)

   right_index = pd.Index(['K0', 'K1', 'K2', 'K2'], name='key1')

   right = pd.DataFrame({'C': ['C0', 'C1', 'C2', 'C3'],
                         'D': ['D0', 'D1', 'D2', 'D3'],
                         'key2': ['K0', 'K0', 'K0', 'K1']},
                        index=right_index)

   result = left.merge(right, on=['key1', 'key2'])

.. ipython:: python
   :suppress:

   @savefig merge_on_index_and_column.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Note

When DataFrames are merged on a string that matches an index level in both frames, the index level is preserved as an index level in the resulting DataFrame.

Note

When DataFrames are merged using only some of the levels of a MultiIndex, the extra levels will be dropped from the resulting merge. In order to preserve those levels, use reset_index on those level names to move those levels to columns prior to doing the merge.

Note

If a string matches both a column name and an index level name, then a warning is issued and the column takes precedence. This will result in an ambiguity error in a future version.

Overlapping value columns

The merge suffixes argument takes a tuple of list of strings to append to overlapping column names in the input ``DataFrame``s to disambiguate the result columns:

.. ipython:: python

   left = pd.DataFrame({'k': ['K0', 'K1', 'K2'], 'v': [1, 2, 3]})
   right = pd.DataFrame({'k': ['K0', 'K0', 'K3'], 'v': [4, 5, 6]})

   result = pd.merge(left, right, on='k')

.. ipython:: python
   :suppress:

   @savefig merging_merge_overlapped.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

.. ipython:: python

   result = pd.merge(left, right, on='k', suffixes=['_l', '_r'])

.. ipython:: python
   :suppress:

   @savefig merging_merge_overlapped_suffix.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

:meth:`DataFrame.join` has lsuffix and rsuffix arguments which behave similarly.

.. ipython:: python

   left = left.set_index('k')
   right = right.set_index('k')
   result = left.join(right, lsuffix='_l', rsuffix='_r')

.. ipython:: python
   :suppress:

   @savefig merging_merge_overlapped_multi_suffix.png
   p.plot([left, right], result,
          labels=['left', 'right'], vertical=False);
   plt.close('all');

Joining multiple DataFrame or Panel objects

A list or tuple of DataFrames can also be passed to :meth:`~DataFrame.join` to join them together on their indexes.

.. ipython:: python

   right2 = pd.DataFrame({'v': [7, 8, 9]}, index=['K1', 'K1', 'K2'])
   result = left.join([right, right2])

.. ipython:: python
   :suppress:

   @savefig merging_join_multi_df.png
   p.plot([left, right, right2], result,
          labels=['left', 'right', 'right2'], vertical=False);
   plt.close('all');

Merging together values within Series or DataFrame columns

Another fairly common situation is to have two like-indexed (or similarly indexed) Series or DataFrame objects and wanting to "patch" values in one object from values for matching indices in the other. Here is an example:

.. ipython:: python

   df1 = pd.DataFrame([[np.nan, 3., 5.], [-4.6, np.nan, np.nan],
                      [np.nan, 7., np.nan]])
   df2 = pd.DataFrame([[-42.6, np.nan, -8.2], [-5., 1.6, 4]],
                      index=[1, 2])

For this, use the :meth:`~DataFrame.combine_first` method:

.. ipython:: python

   result = df1.combine_first(df2)

.. ipython:: python
   :suppress:

   @savefig merging_combine_first.png
   p.plot([df1, df2], result,
          labels=['df1', 'df2'], vertical=False);
   plt.close('all');

Note that this method only takes values from the right DataFrame if they are missing in the left DataFrame. A related method, :meth:`~DataFrame.update`, alters non-NA values inplace:

.. ipython:: python
   :suppress:

   df1_copy = df1.copy()

.. ipython:: python

   df1.update(df2)

.. ipython:: python
   :suppress:

   @savefig merging_update.png
   p.plot([df1_copy, df2], df1,
          labels=['df1', 'df2'], vertical=False);
   plt.close('all');

Timeseries friendly merging

Merging Ordered Data

A :func:`merge_ordered` function allows combining time series and other ordered data. In particular it has an optional fill_method keyword to fill/interpolate missing data:

.. ipython:: python

   left = pd.DataFrame({'k': ['K0', 'K1', 'K1', 'K2'],
                        'lv': [1, 2, 3, 4],
                        's': ['a', 'b', 'c', 'd']})

   right = pd.DataFrame({'k': ['K1', 'K2', 'K4'],
                         'rv': [1, 2, 3]})

   pd.merge_ordered(left, right, fill_method='ffill', left_by='s')

Merging AsOf

.. versionadded:: 0.19.0

A :func:`merge_asof` is similar to an ordered left-join except that we match on nearest key rather than equal keys. For each row in the left DataFrame, we select the last row in the right DataFrame whose on key is less than the left's key. Both DataFrames must be sorted by the key.

Optionally an asof merge can perform a group-wise merge. This matches the by key equally, in addition to the nearest match on the on key.

For example; we might have trades and quotes and we want to asof merge them.

.. ipython:: python

   trades = pd.DataFrame({
       'time': pd.to_datetime(['20160525 13:30:00.023',
                               '20160525 13:30:00.038',
                               '20160525 13:30:00.048',
                               '20160525 13:30:00.048',
                               '20160525 13:30:00.048']),
       'ticker': ['MSFT', 'MSFT',
                  'GOOG', 'GOOG', 'AAPL'],
       'price': [51.95, 51.95,
                 720.77, 720.92, 98.00],
       'quantity': [75, 155,
                    100, 100, 100]},
       columns=['time', 'ticker', 'price', 'quantity'])

   quotes = pd.DataFrame({
       'time': pd.to_datetime(['20160525 13:30:00.023',
                               '20160525 13:30:00.023',
                               '20160525 13:30:00.030',
                               '20160525 13:30:00.041',
                               '20160525 13:30:00.048',
                               '20160525 13:30:00.049',
                               '20160525 13:30:00.072',
                               '20160525 13:30:00.075']),
       'ticker': ['GOOG', 'MSFT', 'MSFT',
                  'MSFT', 'GOOG', 'AAPL', 'GOOG',
                  'MSFT'],
       'bid': [720.50, 51.95, 51.97, 51.99,
               720.50, 97.99, 720.50, 52.01],
       'ask': [720.93, 51.96, 51.98, 52.00,
               720.93, 98.01, 720.88, 52.03]},
       columns=['time', 'ticker', 'bid', 'ask'])

.. ipython:: python

   trades
   quotes

By default we are taking the asof of the quotes.

.. ipython:: python

   pd.merge_asof(trades, quotes,
                 on='time',
                 by='ticker')

We only asof within 2ms between the quote time and the trade time.

.. ipython:: python

   pd.merge_asof(trades, quotes,
                 on='time',
                 by='ticker',
                 tolerance=pd.Timedelta('2ms'))

We only asof within 10ms between the quote time and the trade time and we exclude exact matches on time. Note that though we exclude the exact matches (of the quotes), prior quotes do propagate to that point in time.

.. ipython:: python

   pd.merge_asof(trades, quotes,
                 on='time',
                 by='ticker',
                 tolerance=pd.Timedelta('10ms'),
                 allow_exact_matches=False)

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

merging.rst

merging.rst

Merge, join, and concatenate

Concatenating objects

Set logic on the other axes

Concatenating using `append`

Ignoring indexes on the concatenation axis

Concatenating with mixed ndims

More concatenating with group keys

Appending rows to a DataFrame

Database-style DataFrame joining/merging

Brief primer on merge methods (relational algebra)

Checking for duplicate keys

The merge indicator

Merge Dtypes

Joining on index

Joining key columns on an index

Joining a single Index to a Multi-index

Joining with two multi-indexes

Merging on a combination of columns and index levels

Overlapping value columns

Joining multiple DataFrame or Panel objects

Merging together values within Series or DataFrame columns

Timeseries friendly merging

Merging Ordered Data

Merging AsOf

Observation Origin	`_merge` value
Merge key only in `'left'` frame	`left_only`
Merge key only in `'right'` frame	`right_only`
Merge key in both frames	`both`

Files

merging.rst

Latest commit

History

merging.rst

File metadata and controls

Merge, join, and concatenate

Concatenating objects

Set logic on the other axes

Concatenating using append

Ignoring indexes on the concatenation axis

Concatenating with mixed ndims

More concatenating with group keys

Appending rows to a DataFrame

Database-style DataFrame joining/merging

Brief primer on merge methods (relational algebra)

Checking for duplicate keys

The merge indicator

Merge Dtypes

Joining on index

Joining key columns on an index

Joining a single Index to a Multi-index

Joining with two multi-indexes

Merging on a combination of columns and index levels

Overlapping value columns

Joining multiple DataFrame or Panel objects

Merging together values within Series or DataFrame columns

Timeseries friendly merging

Merging Ordered Data

Merging AsOf

Concatenating using `append`