What is Pandas groupby and merge?

Pandas groupby and merge means groupby summarizes data by category, and merge combines two DataFrames using a common column.

In real programs, this topic helps in class-wise summaries. Learn the idea first, then type the program yourself and compare the output.

Why should you learn this?

• It is useful for class-wise summaries.
• It connects with combining related tables.
• It improves your ability to read, write and debug Python programs.

Important Terms

These terms are used directly in this lesson. Understand them before memorising the code.

Syntax / Basic Pattern

The simple pattern is: prepare data, apply the concept, then show the result.

import pandas as pd
df = pd.DataFrame({
    "Class": ["X", "X", "XI"],
    "Marks": [80, 90, 85]
})
summary = df.groupby("Class")["Marks"].mean()
print(summary)

Complete Example Program

import pandas as pd

df = pd.DataFrame({
    "Class": ["X", "X", "XI"],
    "Marks": [80, 90, 85]
})

summary = df.groupby("Class")["Marks"].mean()
print(summary)

Expected Output

Program Explanation

• import pandas as pd imports ready-made features from a module/library.
• df = pd.DataFrame({ stores a value in df.
• "Class": ["X", "X", "XI"], performs the next step of the program logic.
• "Marks": [80, 90, 85] performs the next step of the program logic.
• }) performs the next step of the program logic.
• summary = df.groupby("Class")["Marks"].mean() stores a value in summary.
• print(summary) displays information or calculated result on the screen.

Where will you use it?

• Class-wise summaries.
• Combining related tables.
• Category-wise analysis.

Common Mistakes

• Analysing data before checking missing values, duplicates and data types.
• Changing original data without keeping a clean copy.
• Creating charts without title, labels or explanation.

Practice Tasks

1. Type the program in pandas-groupby-merge.py and run it.
2. Change input values or sample data and observe the new output.
3. Create one example related to class-wise summaries.
4. Write 5 lines explaining the logic in your own words.

Summary

Pandas groupby and merge is not a theory-only topic. You should be able to explain the meaning, write the example, run it successfully, and use it in a small practical program.