Performing a left join between two DataFrames in Pandas

Last update on April 09 2025 12:52:13 (UTC/GMT +8 hours)

3. Left Join Merge

Write a Pandas program that performs a left join of two DataFrames.

This exercise demonstrates a left join of two DataFrames, including all rows from the left DataFrame and matching rows from the right.

Sample Solution :

Code :

import pandas as pd

# Create two sample DataFrames
df1 = pd.DataFrame({
    'ID': [1, 2, 3],
    'Name': ['Selena', 'Annabel', 'Caeso']
})

df2 = pd.DataFrame({
    'ID': [2, 3, 4],
    'Age': [25, 30, 22]
})

# Perform a left join on the 'ID' column
left_joined_df = pd.merge(df1, df2, on='ID', how='left')

# Output the result
print(left_joined_df)

Output:

   ID     Name   Age
0   1   Selena   NaN
1   2  Annabel  25.0
2   3    Caeso  30.0               

Explanation:

• Created two DataFrames df1 and df2.
• Used pd.merge() with how='left' to perform a left join.
• The result includes all rows from the left DataFrame and only the matching rows from the right DataFrame.

For more Practice: Solve these Related Problems:

• Write a Pandas program to perform a left join on two DataFrames and replace missing values from the right DataFrame with the left’s column median.
• Write a Pandas program to perform a left join on two DataFrames and then verify that all rows from the left DataFrame are preserved.
• Write a Pandas program to perform a left join on two DataFrames and sort the resulting DataFrame based on a column from the right DataFrame.
• Write a Pandas program to perform a left join on two DataFrames and then group the merged result by a column from the left DataFrame.

Python-Pandas Code Editor:

Have another way to solve this solution? Contribute your code (and comments) through Disqus.