GROUP BY

GROUP BY is what makes aggregate functions truly powerful. It groups rows with the same value and lets you calculate aggregates per group.

Without GROUP BY

SELECT COUNT(*) FROM products;
-- Returns: 5 (total count of ALL products)

With GROUP BY

SELECT category, COUNT(*) FROM products
GROUP BY category;
-- Returns: count PER category
-- Electronics  2
-- Office        2
-- Books         1

How It Works

• SQL looks at the GROUP BY column
• Groups all rows with the same value together
• Applies the aggregate function to each group separately

💡 Real-World Examples

• "How many orders per customer?" → GROUP BY customer_id
• "Total sales per month?" → GROUP BY month
• "Average salary per department?" → GROUP BY department

⚠️ Important Rule

Every column in SELECT must either be:

• In the GROUP BY clause, OR
• Inside an aggregate function (COUNT, SUM, AVG, etc.)

Your Task

Count how many products are in each category.