dataframe_合并

（1）merge

df1.merge(df2,on="intopiece",how="outer")   == pd.merge(df1,df2,on="intopiece",how="outer")

--只能合并两个dataframe

--两个dataframe至少有一列相同

df1 = pd.DataFrame({'key1':['a','b','c','d'],'key3':['i','j','k','l']},index=['k','l','m','n',])
df2 = pd.DataFrame({'key1':['a','B','c','d'],'key2':['e','f','g','H']},index = ['k','q','u','v'])
print(df1)
print(df2)
print(pd.merge(df1,df2,how="outer"))
print(df1.merge(df2,how="outer"))

  key1 key3
k    a    i
l    b    j
m    c    k
n    d    l
  key1 key2
k    a    e
q    B    f
u    c    g
v    d    H
  key1 key3 key2
0    a    i    e
1    b    j  NaN
2    c    k    g
3    d    l    H
4    B  NaN    f
  key1 key3 key2
0    a    i    e
1    b    j  NaN
2    c    k    g
3    d    l    H
4    B  NaN    f

（2）join

df1.join(df2,how="left")  

--只能合并两个dataframe

--两个dataframe索引相同

df1 = pd.DataFrame({'key1':['a','b','c','d'],'key3':['i','j','k','l']},index=['k','l','m','n',])
df2 = pd.DataFrame({'key4':['a','B','c','d'],'key2':['e','f','g','H']},index = ['k','q','u','v'])
print(df1)
print(df2)
print(df1.join(df2,how="left"))

  key1 key3
k    a    i
l    b    j
m    c    k
n    d    l
  key2 key4
k    e    a
q    f    B
u    g    c
v    H    d
  key1 key3 key2 key4
k    a    i    e    a
l    b    j  NaN  NaN
m    c    k  NaN  NaN
n    d    l  NaN  NaN

（1）concat

 pd.concat([df1,df2],join="outer",axis=1)

--可以合并多个个dataframe

--合并得dataframe索引相同

df1 = pd.DataFrame({'key1':['a','b','c','d'],'key3':['i','j','k','l']},index=['k','l','m','n',])
df2 = pd.DataFrame({'key4':['a','B','c','d'],'key2':['e','f','g','H']},index = ['k','q','u','v'])
print(df1)
print(df2)
print(pd.concat([df1,df2],join="outer",axis=1))


  key1 key3
k    a    i
l    b    j
m    c    k
n    d    l
  key2 key4
k    e    a
q    f    B
u    g    c
v    H    d
  key1 key3 key2 key4
k    a    i    e    a
l    b    j  NaN  NaN
m    c    k  NaN  NaN
n    d    l  NaN  NaN
q  NaN  NaN    f    B
u  NaN  NaN    g    c
v  NaN  NaN    H    d