Part 9: GROUP BY, HAVING, and Aggregations in SQL Server
Microsoft SQL Server Tutorial Series: Beginner to Expert
Welcome to Part 9 of our SQL Server tutorial series! In this lesson, we’ll explore how to summarize and group data using SQL Server’s powerful aggregation functions, GROUP BY and HAVING clauses.
🎯 What You'll Learn
- How to group data using GROUP BY
- How to use aggregate functions like COUNT, SUM, AVG, MIN, MAX
- How to filter grouped results with HAVING
- Practical examples with real-world use cases
- Best practices for performance and clarity
🧠 What is GROUP BY?
The GROUP BY clause groups rows that have the same values in specified columns into summary rows. It's typically used with aggregate functions.
SELECT Column1, AGG_FUNCTION(Column2)
FROM TableName
GROUP BY Column1;Example: Count students per active status:
SELECT IsActive, COUNT(*) AS TotalStudents
FROM Students
GROUP BY IsActive;
📊 Common Aggregate Functions in SQL Server
| Function | Description | Example |
|---|---|---|
COUNT() |
Counts number of rows | COUNT(*) |
SUM() |
Total of a numeric column | SUM(Marks) |
AVG() |
Average value | AVG(Fees) |
MIN() |
Lowest value | MIN(Age) |
MAX() |
Highest value | MAX(Score) |
📌 Using HAVING to Filter Groups
HAVING is like WHERE, but for grouped results.
SELECT CourseID, COUNT(*) AS Enrolled
FROM Enrollments
GROUP BY CourseID
HAVING COUNT(*) > 10;
This returns only courses with more than 10 enrollments.
📚 Real-World Example: Students per Year
SELECT YEAR(BirthDate) AS BirthYear, COUNT(*) AS StudentCount
FROM Students
GROUP BY YEAR(BirthDate)
ORDER BY BirthYear;
This query groups students by their year of birth and counts how many students were born each year.
💡 Best Practices
| Practice | Why It Matters |
|---|---|
| Use aliases for aggregated columns | Improves readability (e.g., COUNT(*) AS Total) |
| Use WHERE before GROUP BY | Filter raw data before grouping for performance |
| Use HAVING for filtering groups only | HAVING is evaluated after GROUP BY |
| Format numbers and dates in SELECT | Use FORMAT() if needed for presentation |
🧾 Quick SQL Cheat Sheet
-- Count rows per group SELECT Department, COUNT(*) AS Total FROM Employees GROUP BY Department; -- Filter groups SELECT Category, SUM(Price) AS TotalSales FROM Products GROUP BY Category HAVING SUM(Price) > 10000;
✅ Summary
- Use
GROUP BYto group data by columns - Apply aggregate functions to summarize data
- Use
HAVINGto filter aggregated results - Combine
WHERE,GROUP BY, andHAVINGfor full power
🔗 What’s Next?
- Part 8: SQL Joins (INNER, LEFT, RIGHT, FULL)
- Up Next → Part 10: Subqueries and Common Table Expressions (CTEs)
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