SQL GROUP BY Clause | Grouping Data with Aggregate Functions

SQL GROUP BY

Videos Discussion

So far, we have learned how to:

Filter records using WHERE

Sort data using ORDER BY

Summarize data using Aggregate Functions like COUNT, SUM, AVG

But there is a common real-world requirement:

“Show summary category-wise, not for the entire table.”

Examples

Total salary per department

Number of employees in each city

Average marks per subject

If we use aggregate functions without grouping, SQL returns only one result for the whole table. To generate summaries group-wise, SQL provides the GROUP BY clause.

What Is the GROUP BY Clause?

The GROUP BY clause is used to group rows that have the same values in one or more columns and then apply aggregate functions on each group.

GROUP BY works as a bridge between raw data and summarized reports.

How GROUP BY Works

1. SQL scans all rows in the table

2. Rows with the same group value are placed together

3. Aggregate functions are applied to each group separately

4. One result row is returned per group

Syntax

SELECT column_name, aggregate_function(column_name)
FROM table_name
GROUP BY column_name;

Example: employees Table

id	name	salary	department
1	Amit	60000	IT
2	Neha	75000	HR
3	Rahul	50000	IT
4	Priya	90000	Finance
5	Karan	45000	HR

Example - 1: Total Salary per Department

Problem: Find the total salary paid in each department.

SELECT department, SUM(salary) AS total_salary
FROM employees
GROUP BY department;

Output

department total_salary
IT 110000
HR 120000
Finance 90000

department	total_salary
IT	110000
HR	120000
Finance	90000

Each department is treated as one group.

Why GROUP BY is Mandatory Here

If we write

SELECT department, SUM(salary)
FROM employees;

SQL will throw an error.

Reason

When using aggregate functions, all non aggregated columns must appear in GROUP BY.

Example - 2: Count Employees per Department

SELECT department, COUNT(*) AS total_employees
FROM employees
GROUP BY department;

Shows how many employees work in each department.

GROUP BY with WHERE Clause

Execution Order

WHERE - filters rows

GROUP BY - groups filtered rows

Aggregate functions - applied to each group

Example: Count IT Employees by Department

SELECT department, COUNT(*)
FROM employees
WHERE department = 'IT'
GROUP BY department;

Filtering happens before grouping.

GROUP BY with Multiple Columns

Why Multiple Columns Are Used

Sometimes grouping by one column is not enough.

Example

Department and city

Year and month

Syntax

SELECT column1, column2, aggregate_function(column)
FROM table_name
GROUP BY column1, column2;

Example: Employees per Department and City

SELECT department, city, COUNT(*)
FROM employees
GROUP BY department, city;

Each unique combination becomes a separate group.

Important Points About GROUP BY

GROUP BY is used with aggregate functions

One result row is returned per group

All non-aggregated columns must be in GROUP BY

WHERE filters rows before grouping

HAVING filters groups after grouping

Two Minute Drill

GROUP BY groups similar values
Used with aggregate functions
One row per group is returned
WHERE filters rows first
HAVING filters grouped results
Non aggregated columns must be in GROUP BY