Why combining DataFrames matters in Pandas

Introduction

Combining DataFrames helps you join different pieces of data together. This lets you see the full picture and find useful insights.

You have sales data from different months and want to see the whole year.

You want to add customer details to their purchase records.

You need to merge survey answers with demographic info.

You want to stack daily logs into one big file for analysis.

You want to compare two sets of data side by side.

Syntax

Pandas

pd.concat([df1, df2], axis=0)
pd.merge(df1, df2, on='key')

pd.concat stacks DataFrames either vertically (rows) or horizontally (columns).

pd.merge joins DataFrames based on matching column values, like a database join.

Examples

This stacks two DataFrames vertically, adding rows from df2 below df1.

Pandas

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2], 'B': [3, 4]})
df2 = pd.DataFrame({'A': [5, 6], 'B': [7, 8]})

result = pd.concat([df1, df2], axis=0)
print(result)

This merges two DataFrames side by side where the 'key' column matches.

Pandas

import pandas as pd

df1 = pd.DataFrame({'key': ['K0', 'K1'], 'A': [1, 2]})
df2 = pd.DataFrame({'key': ['K0', 'K1'], 'B': [3, 4]})

result = pd.merge(df1, df2, on='key')
print(result)

Sample Program

This example stacks January and February sales data into one DataFrame to see total sales over two months.

Pandas

import pandas as pd

# Sales data for January
jan_sales = pd.DataFrame({
    'Product': ['Apple', 'Banana'],
    'Sales': [100, 150]
})

# Sales data for February
feb_sales = pd.DataFrame({
    'Product': ['Apple', 'Banana'],
    'Sales': [120, 130]
})

# Combine sales data for both months
all_sales = pd.concat([jan_sales, feb_sales], axis=0, ignore_index=True)

print(all_sales)

Important Notes

Use ignore_index=True in pd.concat to reset row numbers after combining.

When merging, make sure the key columns have matching names and data types.

Combining data helps avoid working with many small files separately.

Summary

Combining DataFrames lets you join or stack data to get a complete view.

Use pd.concat to stack rows or columns.

Use pd.merge to join DataFrames based on matching columns.

Practice

1. Why do we combine DataFrames in pandas?
pd.concat and pd.merge are common methods. What is the main reason to combine DataFrames?

easy

A. To get a complete view by joining or stacking data

B. To delete duplicate rows automatically

C. To convert DataFrames into lists

D. To change the data types of columns

Solution

1. Step 1: Understand the purpose of combining DataFrames

   Combining DataFrames helps to bring together data from different sources or parts to analyze them as one.

2. Step 2: Identify the main reason

   Using pd.concat stacks data vertically or horizontally, and pd.merge joins data based on matching columns, both to get a fuller dataset.

3. Final Answer:

   To get a complete view by joining or stacking data -> Option A

4. Quick Check:

   Combining DataFrames = Complete view [OK]

Hint: Combining means joining or stacking data for full info [OK]

Common Mistakes:

• Thinking combining deletes duplicates automatically
• Confusing combining with data type changes
• Believing combining converts DataFrames to lists

2. Which of the following is the correct syntax to stack two DataFrames df1 and df2 vertically using pandas?

easy

A. pd.merge(df1, df2, axis=0)

B. pd.concat([df1, df2], axis=0)

C. df1.append(df2, axis=1)

D. pd.concat(df1, df2, axis=1)

Solution

1. Step 1: Recall the function to stack DataFrames vertically

   pd.concat stacks DataFrames; axis=0 means stacking rows vertically.

2. Step 2: Check the syntax

   The correct syntax is pd.concat([df1, df2], axis=0). Other options misuse pd.merge or wrong axis or argument types.

3. Final Answer:

   pd.concat([df1, df2], axis=0) -> Option B

4. Quick Check:

   Stack vertically = concat with axis=0 [OK]

Hint: Use pd.concat with a list and axis=0 to stack rows [OK]

Common Mistakes:

• Using pd.merge instead of pd.concat for stacking
• Passing DataFrames without a list to pd.concat
• Using wrong axis value for vertical stacking

3. What will be the output of this code?

import pandas as pd

df1 = pd.DataFrame({'key': ['A', 'B'], 'val1': [1, 2]})
df2 = pd.DataFrame({'key': ['B', 'C'], 'val2': [3, 4]})
result = pd.merge(df1, df2, on='key', how='inner')
print(result)

medium

A. key val1
0 A 1
1 B 2

B. key val1 val2
0 A 1 3
1 B 2 4

C. key val1 val2
0 A 1 NaN
1 B 2 3
2 C NaN 4

D. key val1 val2
0 B 2 3

Solution

1. Step 1: Understand pd.merge with how='inner'

   Inner join keeps only rows with keys present in both DataFrames. Here keys are 'A', 'B' in df1 and 'B', 'C' in df2.

2. Step 2: Find common keys and merge rows

   Only 'B' is common. So result has one row with key 'B', val1=2 from df1, val2=3 from df2.

3. Final Answer:

   key val1 val2 0 B 2 3 -> Option D

4. Quick Check:

   Inner join = common keys only [OK]

Hint: Inner merge keeps only matching keys in both DataFrames [OK]

Common Mistakes:

• Expecting all keys from both DataFrames (confusing with outer join)
• Mixing up val2 values for keys
• Ignoring how='inner' effect

4. The following code throws an error. What is the main issue?

import pandas as pd

df1 = pd.DataFrame({'A': [1, 2]})
df2 = pd.DataFrame({'B': [3, 4]})
result = pd.merge(df1, df2, on='A')
print(result)

medium

A. Column 'A' does not exist in both DataFrames

B. pd.merge cannot merge DataFrames with different columns

C. pd.merge requires axis parameter

D. DataFrames must have same number of rows to merge

Solution

1. Step 1: Check columns in both DataFrames

   df1 has column 'A', but df2 has only column 'B'. So 'A' is missing in df2.

2. Step 2: Understand pd.merge on parameter

   pd.merge requires the 'on' column to be present in both DataFrames to join on it. Missing column causes error.

3. Final Answer:

   Column 'A' does not exist in both DataFrames -> Option A

4. Quick Check:

   Merge 'on' column must be in both DataFrames [OK]

Hint: Merge 'on' column must exist in both DataFrames [OK]

Common Mistakes:

• Thinking axis parameter is needed for merge
• Believing DataFrames must have same row count
• Assuming merge works with different columns without 'on'

5. You have two DataFrames:

df1 = pd.DataFrame({'id': [1, 2, 3], 'score': [90, 80, 70]})
df2 = pd.DataFrame({'id': [2, 3, 4], 'grade': ['B', 'C', 'A']})

You want a DataFrame that includes all ids from both, with scores and grades matched where possible, and NaN where missing. Which code achieves this?

hard

A. pd.concat([df1, df2], axis=0)

B. pd.concat([df1, df2], axis=1)

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

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

Solution

1. Step 1: Understand the goal

   We want all ids from both DataFrames, matching scores and grades where ids overlap, and NaN where data is missing.

2. Step 2: Choose the correct merge type

   Using how='outer' in pd.merge keeps all keys from both DataFrames, filling missing values with NaN.

3. Step 3: Check other options

   pd.concat stacks data but does not join on keys; how='inner' drops ids not in both.

4. Final Answer:

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

5. Quick Check:

   Outer merge = all keys, fill missing with NaN [OK]

Hint: Use outer merge to keep all keys and fill missing data [OK]

Common Mistakes:

• Using inner merge and losing unmatched ids
• Using concat which stacks instead of joining
• Confusing axis parameter with merge how