Python数据分析入门（十一）：数据合并

淡淡的烟草味﹌ 2022-11-16 00:58 385阅读 0赞

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# [Python学习交流群：1039649593][Python_1039649593] #

# 数据合并(pd.merge) #

*  根据单个或多个键将不同DataFrame的行连接起来
 *  类似数据库的连接操作
 *  pd.merge:(left, right, how='inner',on=None,left\_on=None, right\_on=None )  
    left:合并时左边的DataFrame  
    right:合并时右边的DataFrame  
    how:合并的方式,默认'inner', 'outer', 'left', 'right'  
    on:需要合并的列名,必须两边都有的列名，并以 left 和 right 中的列名的交集作为连接键  
    left\_on: left Dataframe中用作连接键的列  
    right\_on: right Dataframe中用作连接键的列
 *  内连接 inner:对两张表都有的键的交集进行联合

![f4ba1d3cd86a77f373fa0d58e6324fcb.png][]

*  全连接 outer：对两者表的都有的键的并集进行联合
    
     
    
    ![32f3424ed5c8d1bb8c0a2244978166fc.png][]
 *  左连接 left：对所有左表的键进行联合
    
     
    
    ![339fbc94f52eb5e44f3c1a8fa21a4020.png][]
 *  右连接 right：对所有右表的键进行联合
    
     
    
    ![b71ed26ffdb900bbf75406589bb13bd8.png][]
    
     
    
    示例代码：

import pandas as pd
    import numpy as np
    
    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']})
    
    pd.merge(left,right,on='key') #指定连接键key

运行结果：

key    A    B    C    D
    0    K0    A0    B0    C0    D0
    1    K1    A1    B1    C1    D1
    2    K2    A2    B2    C2    D2
    3    K3    A3    B3    C3    D3

![d2101d611513153283ed312049108360.png][]

示例代码：

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']})
    
    pd.merge(left,right,on=['key1','key2']) #指定多个键，进行合并

运行结果：

key1    key2    A    B    C    D
    0    K0    K0    A0    B0    C0    D0
    1    K1    K0    A2    B2    C1    D1
    2    K1    K0    A2    B2    C2    D2

![db7d9fcc2b050ad2035db1ec1b79742b.png][]

#指定左连接
    
    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']})
    
    pd.merge(left, right, how='left', on=['key1', 'key2'])
        key1    key2          A    B    C    D
    0    K0        K0        A0    B0    C0    D0
    1    K0        K1        A1    B1    NaN    NaN
    2    K1        K0        A2    B2    C1    D1
    3    K1        K0        A2    B2    C2    D2
    4    K2        K1        A3    B3    NaN    NaN

![84cafee3eeca2703c8bcf8a966a379c9.png][]

#指定右连接
    
    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']})
    pd.merge(left, right, how='right', on=['key1', 'key2'])
        key1    key2          A    B    C    D
    0    K0        K0        A0    B0    C0    D0
    1    K1        K0        A2    B2    C1    D1
    2    K1        K0        A2    B2    C2    D2
    3    K2        K0        NaN    NaN    C3    D3

![cd9de7d5dd7c80d8291b0c32d93f7e74.png][]

默认是“内连接”(inner)，即结果中的键是交集

how指定连接方式

### “外连接”(outer)，结果中的键是并集 ###

示例代码：

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

运行结果：

key1    key2    A    B    C    D
    0    K0    K0    A0    B0    C0    D0
    1    K0    K1    A1    B1    NaN    NaN
    2    K1    K0    A2    B2    C1    D1
    3    K1    K0    A2    B2    C2    D2
    4    K2    K1    A3    B3    NaN    NaN
    5    K2    K0    NaN    NaN    C3    D3

![e90230049c6e13e28353ae2d56741866.png][]

### 处理重复列名 ###

参数suffixes：默认为\_x, \_y

示例代码：

# 处理重复列名
    df_obj1 = pd.DataFrame({'key': ['b', 'b', 'a', 'c', 'a', 'a', 'b'],
                            'data' : np.random.randint(0,10,7)})
    df_obj2 = pd.DataFrame({'key': ['a', 'b', 'd'],
                            'data' : np.random.randint(0,10,3)})
    
    print(pd.merge(df_obj1, df_obj2, on='key', suffixes=('_left', '_right')))

运行结果：

data_left key  data_right
    0          9   b           1
    1          5   b           1
    2          1   b           1
    3          2   a           8
    4          2   a           8
    5          5   a           8

### 按索引连接 ###

参数left\_index=True或right\_index=True

示例代码：

# 按索引连接
    df_obj1 = pd.DataFrame({'key': ['b', 'b', 'a', 'c', 'a', 'a', 'b'],
                            'data1' : np.random.randint(0,10,7)})
    df_obj2 = pd.DataFrame({'data2' : np.random.randint(0,10,3)}, index=['a', 'b', 'd'])
    
    print(pd.merge(df_obj1, df_obj2, left_on='key', right_index=True))

运行结果：

data1 key  data2
    0      3   b      6
    1      4   b      6
    6      8   b      6
    2      6   a      0
    4      3   a      0
    5      0   a      0

# 数据合并(pd.concat) #

沿轴方向将多个对象合并到一起

### 1. NumPy的concat ###

np.concatenate

示例代码：

import numpy as np
    import pandas as pd
    
    arr1 = np.random.randint(0, 10, (3, 4))
    arr2 = np.random.randint(0, 10, (3, 4))
    
    print(arr1)
    print(arr2)
    
    print(np.concatenate([arr1, arr2]))
    print(np.concatenate([arr1, arr2], axis=1))

运行结果：

# print(arr1)
    [[3 3 0 8]
     [2 0 3 1]
     [4 8 8 2]]
    
    # print(arr2)
    [[6 8 7 3]
     [1 6 8 7]
     [1 4 7 1]]
    
    # print(np.concatenate([arr1, arr2]))
     [[3 3 0 8]
     [2 0 3 1]
     [4 8 8 2]
     [6 8 7 3]
     [1 6 8 7]
     [1 4 7 1]]
    
    # print(np.concatenate([arr1, arr2], axis=1)) 
    [[3 3 0 8 6 8 7 3]
     [2 0 3 1 1 6 8 7]
     [4 8 8 2 1 4 7 1]]

### 2. pd.concat ###

*  注意指定轴方向，默认axis=0
 *  join指定合并方式，默认为outer
 *  Series合并时查看行索引有无重复

df1 = pd.DataFrame(np.arange(6).reshape(3,2),index=list('abc'),columns=['one','two'])
    
    df2 = pd.DataFrame(np.arange(4).reshape(2,2)+5,index=list('ac'),columns=['three','four'])
    
    pd.concat([df1,df2]) #默认外连接，axis=0
        four    one    three    two
    a    NaN        0.0    NaN        1.0
    b    NaN        2.0    NaN        3.0
    c    NaN        4.0    NaN        5.0
    a    6.0        NaN    5.0        NaN
    c    8.0        NaN    7.0        NaN
    
    pd.concat([df1,df2],axis='columns') #指定axis=1连接
        one    two    three    four
    a    0    1    5.0        6.0
    b    2    3    NaN        NaN
    c    4    5    7.0        8.0
    
    #同样我们也可以指定连接的方式为inner
    pd.concat([df1,df2],axis=1,join='inner')
    
        one    two    three    four
    a    0    1    5        6
    c    4    5    7        8

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