Merging on different column names in Pandas - Mini Project: Build & Apply

Merging DataFrames on Different Column Names

📖 Scenario: You work in a small store. You have two lists of information. One list has product IDs and names. The other list has product codes and prices. You want to combine these lists to see the product name with its price.

🎯 Goal: Build a program that merges two pandas DataFrames using columns with different names to get a combined table showing product names and prices.

📋 What You'll Learn

Create two pandas DataFrames with given data

Use a merge operation on columns with different names

Display the merged DataFrame with product names and prices

💡 Why This Matters

🌍 Real World

Stores and businesses often have data in separate lists or tables with different column names. Merging on different column names helps combine this data for better analysis.

💼 Career

Data analysts and scientists frequently merge datasets from different sources. Knowing how to merge on different column names is a key skill for data cleaning and preparation.

1

Create the first DataFrame

Create a pandas DataFrame called df_products with these exact columns and values:
product_id: [101, 102, 103]
product_name: ['Pen', 'Notebook', 'Eraser']

Pandas

import pandas as pd
# Create the DataFrame df_products with product_id and product_name columns
# Your code here

Hint

Use pd.DataFrame with a dictionary where keys are column names and values are lists of data.

2

Create the second DataFrame

Create a pandas DataFrame called df_prices with these exact columns and values:
code: [101, 102, 103]
price: [1.5, 2.0, 0.5]

Pandas

import pandas as pd

df_products = pd.DataFrame({
    'product_id': [101, 102, 103],
    'product_name': ['Pen', 'Notebook', 'Eraser']
})
# Create the DataFrame df_prices with code and price columns
# Your code here

Hint

Use pd.DataFrame with a dictionary for the second table, matching the exact column names and values.

3

Merge the DataFrames on different column names

Use pd.merge to merge df_products and df_prices where df_products.product_id matches df_prices.code. Save the result in a variable called df_merged.

Pandas

import pandas as pd

df_products = pd.DataFrame({
    'product_id': [101, 102, 103],
    'product_name': ['Pen', 'Notebook', 'Eraser']
})

df_prices = pd.DataFrame({
    'code': [101, 102, 103],
    'price': [1.5, 2.0, 0.5]
})
# Merge df_products and df_prices where product_id matches code
# Your code here

Hint

Use pd.merge with left_on and right_on parameters to join on columns with different names.

4

Display the merged DataFrame

Print the df_merged DataFrame to show the combined product names and prices.

Pandas

import pandas as pd

df_products = pd.DataFrame({
    'product_id': [101, 102, 103],
    'product_name': ['Pen', 'Notebook', 'Eraser']
})

df_prices = pd.DataFrame({
    'code': [101, 102, 103],
    'price': [1.5, 2.0, 0.5]
})

df_merged = pd.merge(df_products, df_prices, left_on='product_id', right_on='code')
# Print the merged DataFrame
# Your code here

Hint

Use print(df_merged) to show the combined table.

Practice

1.

What does the left_on and right_on parameters do in pandas.merge()?

easy

A. They specify columns with different names to join on from left and right DataFrames.

B. They rename columns in the DataFrames before merging.

C. They filter rows based on column values before merging.

D. They sort the DataFrames before merging.

Solution

1. Step 1: Understand merge parameters

   The left_on and right_on parameters specify which columns to join on when the column names differ between DataFrames.

2. Step 2: Clarify their purpose

   They do not rename or filter data but tell pandas which columns to match for merging.

3. Final Answer:

   They specify columns with different names to join on from left and right DataFrames. -> Option A

4. Quick Check:

   left_on/right_on = specify join columns [OK]

Hint: Use left_on and right_on to join different column names [OK]

Common Mistakes:

• Thinking they rename columns
• Confusing with filtering rows
• Assuming they sort data

2.

Which of the following is the correct syntax to merge df1 and df2 on columns 'id' in df1 and 'user_id' in df2?

df1 = pd.DataFrame({'id': [1, 2], 'val': ['A', 'B']})
df2 = pd.DataFrame({'user_id': [1, 3], 'score': [10, 20]})

easy

A. pd.merge(df1, df2, left_on='user_id', right_on='id')

B. pd.merge(df1, df2, on='id', right_on='user_id')

C. pd.merge(df1, df2, left_on='id', right_on='user_id')

D. pd.merge(df1, df2, on=['id', 'user_id'])

Solution

1. Step 1: Identify correct parameters

   Since columns have different names, use left_on='id' and right_on='user_id' to specify join keys.

2. Step 2: Check syntax correctness

   pd.merge(df1, df2, left_on='id', right_on='user_id') correctly uses pd.merge(df1, df2, left_on='id', right_on='user_id'). Others misuse on or swap keys.

3. Final Answer:

   pd.merge(df1, df2, left_on='id', right_on='user_id') -> Option C

4. Quick Check:

   left_on='id', right_on='user_id' = correct syntax [OK]

Hint: Use left_on for left, right_on for right column names [OK]

Common Mistakes:

• Using on with different column names
• Swapping left_on and right_on
• Passing both columns in on as a list

3.

What is the output of the following code?

import pandas as pd

df1 = pd.DataFrame({'key1': ['A', 'B', 'C'], 'val1': [1, 2, 3]})
df2 = pd.DataFrame({'key2': ['B', 'C', 'D'], 'val2': [4, 5, 6]})

result = pd.merge(df1, df2, left_on='key1', right_on='key2', how='inner')
print(result)

medium

A. key1 val1 key2 val2
0 A 1 A 4
1 B 2 B 5

B. key1 val1 key2 val2
0 A 1 B 4
1 B 2 C 5

C. Empty DataFrame
Columns: [key1, val1, key2, val2]
Index: []

D. key1 val1 key2 val2
0 B 2 B 4
1 C 3 C 5

Solution

1. Step 1: Understand merge keys and type

   We merge on key1 from df1 and key2 from df2 with how='inner', so only matching keys appear.

2. Step 2: Find matching keys

   Matching keys are 'B' and 'C'. Rows with 'A' and 'D' are excluded.

3. Final Answer:

   key1 val1 key2 val2 0 B 2 B 4 1 C 3 C 5 -> Option D

4. Quick Check:

   Inner join on matching keys = rows with B and C [OK]

Hint: Inner merge keeps only matching keys from both sides [OK]

Common Mistakes:

• Expecting all rows to appear
• Mixing up left_on and right_on
• Confusing inner with outer join

4.

What is wrong with this code snippet?

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2], 'B': [3, 4]})
df2 = pd.DataFrame({'X': [1, 3], 'Y': [5, 6]})

merged = pd.merge(df1, df2, left_on='A', right_on='B')
print(merged)

medium

A. It will raise a KeyError because 'B' is not in df2.

B. It uses wrong parameter names for merge.

C. It tries to merge on columns that do not match logically.

D. It merges correctly without errors.

Solution

1. Step 1: Check columns in each DataFrame

   df1 has columns 'A' and 'B'. df2 has columns 'X' and 'Y'. The code tries to merge on left_on='A' and right_on='B'.

2. Step 2: Identify error cause

   Since df2 does not have column 'B', pandas will raise a KeyError.

3. Final Answer:

   It will raise a KeyError because 'B' is not in df2. -> Option A

4. Quick Check:

   right_on column must exist in right DataFrame [OK]

Hint: Check if merge columns exist in respective DataFrames [OK]

Common Mistakes:

• Assuming columns exist in both DataFrames
• Confusing left_on and right_on columns
• Ignoring error messages

5.

You have two DataFrames:

df_customers = pd.DataFrame({'cust_id': [101, 102, 103], 'name': ['Alice', 'Bob', 'Charlie']})
df_orders = pd.DataFrame({'customer': [102, 101, 104], 'order_id': [5001, 5002, 5003]})

You want to merge these to get customer names with their orders, including customers without orders and orders without customers. Which code correctly does this?

hard

A. pd.merge(df_customers, df_orders, left_on='cust_id', right_on='customer', how='inner')

B. pd.merge(df_customers, df_orders, left_on='cust_id', right_on='customer', how='outer')

C. pd.merge(df_customers, df_orders, left_on='name', right_on='customer', how='left')

D. pd.merge(df_customers, df_orders, on='cust_id', how='inner')

Solution

1. Step 1: Identify columns to join on

   Customer IDs are cust_id in df_customers and customer in df_orders. So use left_on='cust_id' and right_on='customer'.

2. Step 2: Choose join type for all data

   To include all customers and all orders (even unmatched), use how='outer' for full outer join.

3. Final Answer:

   pd.merge(df_customers, df_orders, left_on='cust_id', right_on='customer', how='outer') -> Option B

4. Quick Check:

   Outer join + left_on/right_on = all data combined [OK]

Hint: Use how='outer' with left_on/right_on for full merge [OK]

Common Mistakes:

• Using inner join loses unmatched rows
• Merging on wrong columns
• Using name column instead of ID