SQL: Aggregate Functions and GROUP BY

Aggregate functions perform calculations across multiple rows and return a single result. Combined with GROUP BY, they allow you to summarise data — for example, calculating averages, counting records, or finding the highest value within groups.

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Aggregate Functions

Function	Description	Example
COUNT()	Counts the number of rows	COUNT(*), COUNT(column)
SUM()	Adds up all values in a column	SUM(Price)
AVG()	Calculates the arithmetic mean	AVG(Score)
MIN()	Returns the smallest value	MIN(DateOfBirth)
MAX()	Returns the largest value	MAX(Score)

Examples

-- How many students are there?
SELECT COUNT(*) AS TotalStudents
FROM Students;

-- What is the average grade score?
SELECT AVG(Score) AS AverageScore
FROM Grades;

-- What is the highest score?
SELECT MAX(Score) AS HighestScore
FROM Grades;

-- What is the total cost of all products?
SELECT SUM(Price) AS TotalValue
FROM Products;

COUNT(*) vs COUNT(column)

• COUNT(*) counts all rows, including those with NULL values.
• COUNT(column) counts only rows where that column is not NULL.

-- Count all students
SELECT COUNT(*) FROM Students;