Pandas: LEFT OUTER JOIN, Use keys from left dataframe only

Last update on April 03 2025 12:54:52 (UTC/GMT +8 hours)

8. Left Join Using Keys from Left DataFrame

Write a Pandas program to join (left join) the two dataframes using keys from left dataframe only.

Test Data:

data1:
  key1 key2   P   Q
0   K0   K0  P0  Q0
1   K0   K1  P1  Q1
2   K1   K0  P2  Q2
3   K2   K1  P3  Q3

data2:
  key1 key2   R   S
0   K0   K0  R0  S0
1   K1   K0  R1  S1
2   K1   K0  R2  S2
3   K2   K0  R3  S3

Sample Solution:

Python Code :

import pandas as pd
data1 = pd.DataFrame({'key1': ['K0', 'K0', 'K1', 'K2'],
                     'key2': ['K0', 'K1', 'K0', 'K1'],
                     'P': ['P0', 'P1', 'P2', 'P3'],
                     'Q': ['Q0', 'Q1', 'Q2', 'Q3']}) 
data2 = pd.DataFrame({'key1': ['K0', 'K1', 'K1', 'K2'],
                      'key2': ['K0', 'K0', 'K0', 'K0'],
                      'R': ['R0', 'R1', 'R2', 'R3'],
                      'S': ['S0', 'S1', 'S2', 'S3']})
print("Original DataFrames:")
print(data1)
print("--------------------")
print(data2)
print("\nMerged Data (keys from data1):")
merged_data = pd.merge(data1, data2, how='left', on=['key1', 'key2'])
print(merged_data)
print("\nMerged Data (keys from data2):")
merged_data = pd.merge(data2, data1, how='left', on=['key1', 'key2'])
print(merged_data)

Sample Output:

Original DataFrames:
  key1 key2   P   Q
0   K0   K0  P0  Q0
1   K0   K1  P1  Q1
2   K1   K0  P2  Q2
3   K2   K1  P3  Q3
--------------------
  key1 key2   R   S
0   K0   K0  R0  S0
1   K1   K0  R1  S1
2   K1   K0  R2  S2
3   K2   K0  R3  S3

Merged Data (keys from data1):
  key1 key2   P   Q    R    S
0   K0   K0  P0  Q0   R0   S0
1   K0   K1  P1  Q1  NaN  NaN
2   K1   K0  P2  Q2   R1   S1
3   K1   K0  P2  Q2   R2   S2
4   K2   K1  P3  Q3  NaN  NaN

Merged Data (keys from data2):
  key1 key2   R   S    P    Q
0   K0   K0  R0  S0   P0   Q0
1   K1   K0  R1  S1   P2   Q2
2   K1   K0  R2  S2   P2   Q2
3   K2   K0  R3  S3  NaN  NaN           

For more Practice: Solve these Related Problems:

• Write a Pandas program to perform a left join on two DataFrames using keys from the left DataFrame only and then display the result.
• Write a Pandas program to merge two DataFrames with a left join and then fill missing values in the joined DataFrame.
• Write a Pandas program to join two DataFrames using a left join and then verify that all left DataFrame records are present.
• Write a Pandas program to execute a left merge on two DataFrames and then sort the result by the join key.

Python Code Editor:

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