merge() for SQL-like joins in Pandas

Introduction

We use merge() to combine two tables based on common columns, just like joining tables in a database. It helps us see related data together.

You have two lists of customer info and their orders, and want to see all orders with customer details.

You want to combine sales data from two different months into one table based on product IDs.

You have employee info and their department info in separate tables and want to see full employee details with department names.

You want to find customers who made purchases in both January and February by joining two purchase tables.

Syntax

Pandas

pd.merge(left, right, how='inner', on=None, left_on=None, right_on=None)

left and right are the two tables (DataFrames) to join.

how decides the type of join: 'inner', 'left', 'right', or 'outer'.

Examples

Join two tables on the column 'id' using an inner join (default).

Pandas

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

Keep all rows from df1 and add matching rows from df2.

Pandas

pd.merge(df1, df2, how='left', on='id')

Keep all rows from both tables, filling missing matches with NaN.

Pandas

pd.merge(df1, df2, how='outer', on='id')

Join using different column names in each table.

Pandas

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

Sample Program

This code shows how to join two tables of users and orders using different join types. It prints the results so you can see how rows match or stay when no match is found.

Pandas

import pandas as pd

# Create first table with user info
df_users = pd.DataFrame({
    'user_id': [1, 2, 3],
    'name': ['Alice', 'Bob', 'Charlie']
})

# Create second table with orders
df_orders = pd.DataFrame({
    'order_id': [101, 102, 103, 104],
    'user_id': [2, 2, 3, 4],
    'product': ['Book', 'Pen', 'Notebook', 'Pencil']
})

# Merge tables to see orders with user names
merged_inner = pd.merge(df_users, df_orders, on='user_id')
merged_left = pd.merge(df_users, df_orders, how='left', on='user_id')
merged_outer = pd.merge(df_users, df_orders, how='outer', on='user_id')

print('Inner Join Result:')
print(merged_inner)
print('\nLeft Join Result:')
print(merged_left)
print('\nOuter Join Result:')
print(merged_outer)

Important Notes

If columns have different names, use left_on and right_on to specify them.

Inner join keeps only matching rows; left join keeps all from left table; right join keeps all from right table; outer join keeps all rows from both.

Missing matches fill with NaN, which means 'no data'.

Summary

merge() combines two tables based on matching columns.

You can choose how to join: inner, left, right, or outer.

It helps you see related data from different sources together easily.

Practice

1. What does the merge() function in pandas do?

easy

A. It filters rows based on a condition.

B. It sorts a DataFrame by column values.

C. It changes the data type of a column.

D. It combines two DataFrames based on matching columns.

Solution

1. Step 1: Understand the purpose of merge()

   The merge() function is designed to combine two tables (DataFrames) by matching values in specified columns.

2. Step 2: Compare with other options

   Sorting, filtering, and changing data types are different operations and not the main purpose of merge().

3. Final Answer:

   It combines two DataFrames based on matching columns. -> Option D

4. Quick Check:

   merge() joins tables on columns [OK]

Hint: merge() joins tables by matching columns [OK]

Common Mistakes:

• Confusing merge() with sort or filter functions
• Thinking merge() changes data types
• Assuming merge() works without matching columns

2. Which of the following is the correct syntax to perform an inner join of two DataFrames df1 and df2 on column 'id' using pandas?

easy

A. df1.merge(df2, by='id', join='inner')

B. pd.merge(df1, df2, on='id', how='inner')

C. pd.join(df1, df2, on='id', how='inner')

D. df1.merge(df2, on='id', how='outer')

Solution

1. Step 1: Recall correct merge syntax

   The correct pandas syntax for merging is pd.merge(left, right, on=column, how=type). Here, how='inner' specifies an inner join.

2. Step 2: Check each option

   pd.merge(df1, df2, on='id', how='inner') matches the correct syntax. df1.merge(df2, by='id', join='inner') uses wrong parameter names. pd.join(df1, df2, on='id', how='inner') uses pd.join which does not exist. df1.merge(df2, on='id', how='outer') uses how='outer' which is not inner join.

3. Final Answer:

   pd.merge(df1, df2, on='id', how='inner') -> Option B

4. Quick Check:

   Correct merge syntax = pd.merge(df1, df2, on='id', how='inner') [OK]

Hint: Use pd.merge(df1, df2, on='col', how='inner') [OK]

Common Mistakes:

• Using wrong parameter names like 'by' or 'join'
• Confusing merge() with join() method
• Mixing up 'inner' and 'outer' join types

3. Given the following code:

import pandas as pd

df1 = pd.DataFrame({'id': [1, 2, 3], 'value1': ['A', 'B', 'C']})
df2 = pd.DataFrame({'id': [2, 3, 4], 'value2': ['X', 'Y', 'Z']})
result = pd.merge(df1, df2, on='id', how='inner')
print(result)

What will be the output?

medium

A. id value1 value2
0 2 B X
1 3 C Y

B. id value1 value2
0 1 A NaN
1 2 B X
2 3 C Y

C. id value1 value2
0 2 B X
1 3 C Y
2 4 NaN Z

D. id value1
0 1 A
1 2 B
2 3 C

Solution

1. Step 1: Understand inner join behavior

   An inner join keeps only rows with matching 'id' values in both DataFrames. Here, common ids are 2 and 3.

2. Step 2: Check matching rows and columns

   Rows with id 2 and 3 appear in both df1 and df2, so their columns combine. Rows with id 1 or 4 are excluded.

3. Final Answer:

   id value1 value2 0 2 B X 1 3 C Y -> Option A

4. Quick Check:

   Inner join keeps only matching ids [OK]

Hint: Inner join keeps only rows with matching keys [OK]

Common Mistakes:

• Expecting all rows from both DataFrames
• Confusing inner join with left or outer join
• Ignoring NaN values in output

4. You run this code but get a KeyError:

import pandas as pd

df1 = pd.DataFrame({'key1': [1, 2], 'val': ['A', 'B']})
df2 = pd.DataFrame({'key2': [1, 2], 'val': ['X', 'Y']})
result = pd.merge(df1, df2, on='key1', how='inner')

What is the cause of the error?

medium

A. Column 'key1' does not exist in df2.

B. The merge function requires 'how' to be 'outer'.

C. DataFrames must have the same number of rows to merge.

D. You cannot merge on columns with different names.

Solution

1. Step 1: Check columns in both DataFrames

   df1 has column 'key1', but df2 has 'key2'. The merge tries to join on 'key1' which df2 lacks.

2. Step 2: Understand KeyError cause

   Because 'key1' is missing in df2, pandas raises a KeyError indicating the column is not found.

3. Final Answer:

   Column 'key1' does not exist in df2. -> Option A

4. Quick Check:

   Merge keys must exist in both DataFrames [OK]

Hint: Ensure join columns exist in both DataFrames [OK]

Common Mistakes:

• Assuming merge works with different column names without specifying
• Thinking 'how' parameter causes KeyError
• Believing row counts must match to merge

5. You have two DataFrames:

df_customers = pd.DataFrame({'customer_id': [1, 2, 3], 'name': ['Alice', 'Bob', 'Charlie']})
df_orders = pd.DataFrame({'order_id': [101, 102, 103], 'customer_id': [2, 2, 4], 'amount': [250, 125, 300]})

You want to create a DataFrame showing all customers and their orders, including customers with no orders and orders with no matching customer. Which merge() call achieves this?

hard

A. pd.merge(df_customers, df_orders, on='customer_id', how='left')

B. pd.merge(df_customers, df_orders, on='customer_id', how='inner')

C. pd.merge(df_customers, df_orders, on='customer_id', how='outer')

D. pd.merge(df_customers, df_orders, on='customer_id', how='right')

Solution

1. Step 1: Understand join types

   An outer join keeps all rows from both DataFrames, filling missing matches with NaN.

2. Step 2: Apply to problem context

   We want all customers (even without orders) and all orders (even without matching customers). Only outer join includes all rows from both sides.

3. Final Answer:

   pd.merge(df_customers, df_orders, on='customer_id', how='outer') -> Option C

4. Quick Check:

   Outer join includes all rows from both tables [OK]

Hint: Use how='outer' to keep all rows from both DataFrames [OK]

Common Mistakes:

• Using inner join excludes unmatched rows
• Left join excludes orders without customers
• Right join excludes customers without orders