Python Pandas 简明教程

Python Pandas - Merging/Joining

Pandas 具有功能全面、性能卓越的内存中连接操作，与 SQL 等关系数据库极为相似。

Pandas has full-featured, high performance in-memory join operations idiomatically very similar to relational databases like SQL.

Pandas 提供了一个函数 merge ，作为 DataFrame 对象之间所有标准数据库连接操作的入口点 -

Pandas provides a single function, 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)

此处，我们使用了以下参数 -

Here, we have used the following parameters −

left − A DataFrame object.
right − Another DataFrame object.
on − Columns (names) to join on. Must be found in both the left and right DataFrame objects.
left_on − Columns from the left DataFrame to use as keys. Can either be column names or arrays with length equal to the length of the DataFrame.
right_on − Columns from the right DataFrame to use as keys. Can either be column 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 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. Each method has been described below.
sort − Sort the result DataFrame by the join keys in lexicographical order. Defaults to True, setting to False will improve the performance substantially in many cases.

现在让我们创建两个不同的 DataFrame 并对它们执行合并操作。

Let us now create two different DataFrames and perform the merging operations on it.

# import the pandas library
import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame(
   {'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print left
print right

它的 output 如下所示 −

Its output is as follows −

    Name  id   subject_id
0   Alex   1         sub1
1    Amy   2         sub2
2  Allen   3         sub4
3  Alice   4         sub6
4  Ayoung  5         sub5

    Name  id   subject_id
0  Billy   1         sub2
1  Brian   2         sub4
2  Bran    3         sub3
3  Bryce   4         sub6
4  Betty   5         sub5

Merge Two DataFrames on a Key

import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame({
	'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print pd.merge(left,right,on='id')

它的 output 如下所示 −

Its output is as follows −

   Name_x   id  subject_id_x   Name_y   subject_id_y
0  Alex      1          sub1    Billy           sub2
1  Amy       2          sub2    Brian           sub4
2  Allen     3          sub4     Bran           sub3
3  Alice     4          sub6    Bryce           sub6
4  Ayoung    5          sub5    Betty           sub5

Merge Two DataFrames on Multiple Keys

import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame({
	'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print pd.merge(left,right,on=['id','subject_id'])

它的 output 如下所示 −

Its output is as follows −

    Name_x   id   subject_id   Name_y
0    Alice    4         sub6    Bryce
1   Ayoung    5         sub5    Betty

Merge Using 'how' Argument

merge 的 how 参数指定如何确定要包含在结果表中的键。如果键组合没有出现在左表或右表中，则连接表中的值将为 NA。

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 the right tables, the values in the joined table will be NA.

以下是 how 选项及其 SQL 等价名称的摘要 −

Here is a summary of the how options and their SQL equivalent names −

Merge Method

SQL Equivalent

Description

left

LEFT OUTER JOIN

Use keys from left object

right

RIGHT OUTER JOIN

Use keys from right object

outer

FULL OUTER JOIN

Use union of keys

inner

INNER JOIN

Use intersection of keys

Left Join

import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print pd.merge(left, right, on='subject_id', how='left')

它的 output 如下所示 −

Its output is as follows −

    Name_x   id_x   subject_id   Name_y   id_y
0     Alex      1         sub1      NaN    NaN
1      Amy      2         sub2    Billy    1.0
2    Allen      3         sub4    Brian    2.0
3    Alice      4         sub6    Bryce    4.0
4   Ayoung      5         sub5    Betty    5.0

Right Join

import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print pd.merge(left, right, on='subject_id', how='right')

它的 output 如下所示 −

Its output is as follows −

    Name_x  id_x   subject_id   Name_y   id_y
0      Amy   2.0         sub2    Billy      1
1    Allen   3.0         sub4    Brian      2
2    Alice   4.0         sub6    Bryce      4
3   Ayoung   5.0         sub5    Betty      5
4      NaN   NaN         sub3     Bran      3

Outer Join

import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print pd.merge(left, right, how='outer', on='subject_id')

它的 output 如下所示 −

Its output is as follows −

    Name_x  id_x   subject_id   Name_y   id_y
0     Alex   1.0         sub1      NaN    NaN
1      Amy   2.0         sub2    Billy    1.0
2    Allen   3.0         sub4    Brian    2.0
3    Alice   4.0         sub6    Bryce    4.0
4   Ayoung   5.0         sub5    Betty    5.0
5      NaN   NaN         sub3     Bran    3.0

Inner Join

连接将在索引上执行。连接操作会遵守对其进行调用的对象。因此， a.join(b) 不等于 b.join(a) 。

Joining will be performed on index. Join operation honors the object on which it is called. So, a.join(b) is not equal to b.join(a).

import pandas as pd
left = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Alex', 'Amy', 'Allen', 'Alice', 'Ayoung'],
   'subject_id':['sub1','sub2','sub4','sub6','sub5']})
right = pd.DataFrame({
   'id':[1,2,3,4,5],
   'Name': ['Billy', 'Brian', 'Bran', 'Bryce', 'Betty'],
   'subject_id':['sub2','sub4','sub3','sub6','sub5']})
print pd.merge(left, right, on='subject_id', how='inner')

它的 output 如下所示 −

Its output is as follows −

    Name_x   id_x   subject_id   Name_y   id_y
0      Amy      2         sub2    Billy      1
1    Allen      3         sub4    Brian      2
2    Alice      4         sub6    Bryce      4
3   Ayoung      5         sub5    Betty      5