SQL HAVING clause

Here is a clear, practical, and complete guide to the SQL HAVING clause, including examples and key differences from WHERE.

✅ What the SQL HAVING clause Does

HAVING filters grouped/aggregated results after GROUP BY is applied.

It is similar to WHERE, but:

• WHERE filters rows BEFORE grouping

• HAVING filters groups AFTER aggregation

🔹 Basic Syntax

📌 Simple Examples

1. Departments with more than 10 employees

2. Products with total sales above $1,000

3. Classes where the average score is below 70

🔥 Using HAVING Without GROUP BY

Most databases allow HAVING even without GROUP BY:

Useful when you want to filter by an aggregate without grouping.

🔧 Combine WHERE + HAVING (Best Practice)

Use WHERE first to reduce data before grouping.

Example: Count high-value orders per customer

🧠 Key Difference: WHERE vs HAVING

⚡ Real-World Examples

Find customers who spent over $10,000 total

Find stores with more than 3 employees

Find categories where the average product price is above $30

🏎 Performance Tips

✔ Use WHERE to filter rows early — boosts performance
✔ Use HAVING only for aggregated conditions
✔ Index columns used in WHERE for best performance
✔ Avoid unnecessary HAVING clauses