python中merge函数「建议收藏」

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

>>> a=pd.DataFrame({'key':['a','b','c'],'val1':range(3)})
>>> b=pd.DataFrame({'key':['a','d','a'],'val1':range(3)})

a
  key  val1
0   a     0
1   b     1
2   c     2
 b
  key  val1
0   a     0
1   d     1
2   a     2

 pd.merge(a,b)
key  val1  val2
0   a     0     0
1   a     0     2

pd.merge(a,b,how='outer')         
  key  val1  val2
0   a     0   0.0
1   a     0   2.0
2   b     1   NaN
3   c     2   NaN            

pd.merge(a,b,how='left')
  key  val1  val2
0   a     0   0.0
1   a     0   2.0
2   b     1   NaN
3   c     2   NaN

pd.merge(a,b,how='right')
  key  val1  val2
0   a   0.0     0
1   a   0.0     2
2   d   NaN     1