What is the analogue of EXCEPT clause in SQL in Pandas?

问题

I have a sample pandas dataframe df:

        col1    col2    col3    col4
      0   a      1.0    2.0      3
      1   b      NaN    NaN      6
      2   c      NaN    8.0      9
      3   d      NaN    11.0    12
      4   e     13.0    14.0    15
      5   f     17.0    18.0    19
      6   g     21.0    22.0    23

and a second one df1:

        col1    col2    col3    col4
      0  a      1.0     2.0      3
      4  e     13.0    14.0     15
      5  f     17.0    18.0     19
      6  g     21.0    22.0     23

I want to get the subset of df that does not overlaps with df1. In effect I am looking for the equivalent of the EXCEPT operand in SQL.

I used the subtract() function -- but this was clearly wrong, as the subtract performs elementwise numerical subtraction. So I got an error message:

       TypeError: unsupported operand type(s) for -: 'str' and 'str'

So the question is: What is the equivalent of EXCEPT in SQL for Pandas?

回答1:

I think you need set_index of all string columns first:

df2 = df.set_index('col1').subtract(df1.set_index('col1'), axis='columns')
print (df2)
      col2  col3  col4
col1                  
a      0.0   0.0   0.0
b      NaN   NaN   NaN
c      NaN   NaN   NaN
d      NaN   NaN   NaN
e      0.0   0.0   0.0
f      0.0   0.0   0.0
g      0.0   0.0   0.0

Or:

df2 = df.set_index('col1').subtract(df1.set_index('col1'), axis='columns', fill_value=0)
print (df2)
      col2  col3  col4
col1                  
a      0.0   0.0   0.0
b      NaN   NaN   6.0
c      NaN   8.0   9.0
d      NaN  11.0  12.0
e      0.0   0.0   0.0
f      0.0   0.0   0.0
g      0.0   0.0   0.0

EDIT by edited question:

print (df.isin(df1))
    col1   col2   col3   col4
0   True   True   True   True
1  False  False  False  False
2  False  False  False  False
3  False  False  False  False
4   True   True   True   True
5   True   True   True   True
6   True   True   True   True

print (df.isin(df1).all(axis=1))
0     True
1    False
2    False
3    False
4     True
5     True
6     True
dtype: bool

print (~df.isin(df1).all(axis=1))
0    False
1     True
2     True
3     True
4    False
5    False
6    False
dtype: bool

print (df[~(df.isin(df1).all(axis=1))])
  col1  col2  col3  col4
1    b   NaN   NaN     6
2    c   NaN   8.0     9
3    d   NaN  11.0    12

回答2:

I think a Pandas equivalent for SQL EXCEPT (MINUS) would be the following technique:

In [16]: df1
Out[16]:
   a  b  c
0  1  a  5   # duplicates row with index: 3 
1  0  x  4
2  9  Z  9   # exists in DF2, so it should NOT appear in the result set
3  1  a  5   # duplicates row with index: 3

In [17]: df2
Out[17]:
    a  b    c
0  66  a  5.0
1   9  Z  9.0
2   0  x  NaN

In [18]: (pd.merge(df1, df2, on=df1.columns.tolist(), how='outer', indicator=True)
    ...:    .query("_merge == 'left_only'")
    ...:    .drop('_merge', 1)
    ...: )
    ...:
Out[18]:
   a  b    c
0  1  a  5.0
1  1  a  5.0
2  0  x  4.0

NOTE: this solution does NOT pay attention at indices

来源：https://stackoverflow.com/questions/41483823/what-is-the-analogue-of-except-clause-in-sql-in-pandas

标签

python

sql

pandas

except