✅ SQL Aggregation & GROUP BY Explained 📊🧠

Aggregation functions are used to summarize data, especially when working with grouped values.

1️⃣ Common Aggregate Functions

⦁ COUNT() – Total number of rows
⦁ SUM() – Adds up numeric values
⦁ AVG() – Calculates the average
⦁ MIN() / MAX() – Finds smallest/largest value

Example:

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

➡️ Returns the number of employees in each department.

2️⃣ GROUP BY

Used with aggregate functions to group rows based on column values.

SELECT city, AVG(salary)  
FROM employees  
GROUP BY city;

➡️ Shows average salary by city.

3️⃣ HAVING vs WHERE

⦁ WHERE filters rows before grouping
⦁ HAVING filters groups after aggregation

Example:

SELECT dept_id, COUNT(*)  
FROM employees  
GROUP BY dept_id  
HAVING COUNT(*) > 5;

➡️ Shows departments with more than 5 employees.

4️⃣ GROUP BY Multiple Columns

SELECT dept_id, role, COUNT(*)  
FROM employees  
GROUP BY dept_id, role;

➡️ Groups by department and role for detailed summaries.

5️⃣ Real-World Use Cases

✅ Sales by region
✅ Orders per customer
✅ Avg. rating per product
✅ Monthly revenue reports

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