How to make merge using a few columns in one Data Frame in Python Pandas?

Question

I have two Pandas Data Frames like below:

table1

ID
------
11
22
33
44

table2

ID1    ID2   ID3    VAL
----------------------
11  | 108  | 5    | 5555
45  | 22   | 1146 | 4567
67  | 4987 | 33   | 105

And I need to merge table1 and table2 using column "ID" from table1 and columns: "ID1", "ID2", "ID3" from table2.

I want to do like that:

1. merge on ID and ID1
2. if ID does not merge with ID1 --> merge ID and ID2
3. if ID does not merge with ID1 and ID2 --> merge ID and ID3
4. give NULL or NaN if rows does not merge

So as a result I need table like below:

ID    ID1    ID2    ID3    VAL
-------------------- -----------
11  | 11   | NULL | NULL | 5555
22  | NULL | 22   | NULL | 4567
33  | NULL | NULL | 33   | 105
44  | NULL | NULL | NULL | NULL

How can i do that in Python Pandas ?

Answer 1

for c in df2.columns[:-1]:
    df1[c] = np.where(df1['ID'].eq(df2[c]),df1['ID'],np.nan)
df1 = df1.join(df2.iloc[:,-1])

	ID	ID1	ID2	ID3	VAL
0	11	11	nan	nan	5555
1	22	nan	22	nan	4567
2	33	nan	nan	33	105
3	44	nan	nan	nan	nan