SQL GROUP BY

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The GROUP BY clause in SQL is used to group rows that have the same values in specified columns. It is commonly used with aggregate functions to perform calculations on each group.

Basic Syntax

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

GROUP BY with COUNT(): Count how many students are in each city. 

SELECT city, COUNT(*) 
FROM students
GROUP BY city;

GROUP BY with AVG(): Find the average marks in each city. 

SELECT city, AVG(marks)
FROM students
GROUP BY city;

GROUP BY with Multiple Columns 

SELECT city, marks
FROM students
GROUP BY city, marks;

This groups rows by both city and marks.

Example: Combining GROUP BY and ORDER BY 

SELECT city, COUNT(*)
FROM students
GROUP BY city
ORDER BY city;



This:

Groups students by city
Counts them
Sorts the result by city name

Example: find duplicates is using GROUP BY: 

SELECT email, COUNT(*)
FROM Students
GROUP BY email
HAVING COUNT(*) > 1;

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