2021-08-19

Python中merge()函数的三种连接方式
1.内连接

df1 = pd.DataFrame({'a': ['foo', 'bar'], 'b': [1, 2]})
df2=pd.DataFrame({'a': ['foo', 'baz'], 'c': [3, 4]})
df3=df1.merge(df2,how='inner',on='a')
print(df1)
print(df2)
print(df3)
#输出结果
   a  b
0  foo  1
1  bar  2
     a  c
0  foo  3
1  baz  4
     a  b  c
0  foo  1  3

2.左连接
`在本例中实际上是取df1的全部数据，然后右侧连接df2

df4=df1.merge(df2,how='left',on='a')
print(df4)
 a  b    c
0  foo  1  3.0
1  bar  2  NaN

3.右连接
右连接左连接正好相反

 df5=df1.merge(df2,how='right',on='a')
    a    b  c
0  foo  1.0  3
1  baz  NaN  4
df6=df2.merge(df1,how='right',on='a')
print(df6)
   a    c  b
0  foo  3.0  1
1  bar  NaN  2